Chelate N,O-palladium(ii) complexes: synthesis, characterization and biological activity

Article abstract of DOI:10.1039/c5ra10204a

Four trans chelate N,O-palladium(ii) complexes were synthetized starting from salicylaldehyde anil Schiff bases, as ligands. Their structures were elucidated using experimental and theoretical tools. The structures of the theoretically possible cis isomer

Full text of DOI:10.1039/c5ra10204a

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
View Article Online
View Journal
RSC Advances
This article can be cited before page numbers have been issued, to do this please use: V. P. Petrovi, M. N.
Živanovi, D. Simijonovi, J. orovi, Z. D. Petrovic and S. D. Markovi, RSC Adv., 2015, DOI:
10.1039/C5RA10204A.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. This Accepted Manuscript will be replaced by the edited,
formatted and paginated article as soon as this is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/advances

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Page 1 of 49
RSC Advances
View Article Online
DOI: 10.1039/C5RA10204A
RSC Advances
RSCPublishing
ARTICLE
Chelate N,Oꢀpalladium(II) complexes: synthesis,
characterization and biological activity
Cite this: DOI: 10.1039/x0xx00000x
Vladimir P. Petrović, a Marko N. Živanović,b Dušica Simijonović,a Jelena Đorović,a,c
Zorica D. Petrović, *a and Snežana D. Markovićb
Received 00th January 2012,
Accepted 00th January 2012
The four trans chelate N,Oꢀpalladium(II) complexes were synthetized starting from salicylaldehyde anil
Schiff bases, as ligands. Their structure was elucidated using experimental and theoretical tools. The
structures of the theoretically possible cis isomers are examined using DFT method. The biological
activity, in vitro cytotoxic and prooxidative effects against human breast carcinoma MDAꢀMBꢀ231,
human colon carcinoma HCTꢀ116, and human fibroblast healthy MRCꢀ5 cell lines of investigated
compounds were determined. Schiff bases show moderate or weak cytotoxic effect. On the other hand,
complexes Pdꢀ1 and Pdꢀ6 show significant cytotoxic effect on all three cell lines, with IC50 values in
range of 0.6 to 17.1 µM on HCTꢀ116 cells, 7.2 to 55.6 µM on MDAꢀMBꢀ231 cells and 34.5 to 48.1 µM
on MRCꢀ5 cells. Also, Pdꢀ1 and Pdꢀ6 induce extreme oxidative stress in the all treated cell lines. At this
stage of investigations, Pdꢀ1 and Pdꢀ6 showed no selectivity towards cancer cells, i.e. they were also
cytotoxic to MRCꢀ5 cells in the similar extent. Taking into account these facts, it could be further
investigated how the most active substances impact on the type of the cell death (apoptotic and/or
necrotic pathways).
DOI: 10.1039/x0xx00000x
www.rsc.org/
synthesis, stability and wide application.6 A large number of
Schiff base complexes have been reported so far, and their
Introduction
Azomethines, also known as Schiff bases, are important class
of organic compounds.1 If these compounds contain aniline
moiety (or substituted aniline), they are get the anil in the part
of their names.1 Some of these compounds have wide
applications in organic synthesis, catalysis, analytical
chemistry, food industry, as well as industry of pigments and
dyes.2 Azomethine group, as structural fragment of these
compounds, is present in various natural and synthetic products
and it is responsible for a broad range of biological activities,3
including antibacterial, antifungal, antimalarial, antiꢀ
inflammatory, antiviral, antiproliferative, and antipyretic
properties.3c,4
Moreover, Schiff bases are a special class of ligands with a
variety of donor atoms. If these compounds possess donor
atoms such O, N or S, than they can act as chelating ligands in
designing of many metal complexes. The high affinity of these
compounds for complexation with transition metal ions is
utilized for preparation different complexes.5 Phenolic Schiff
bases as bidentate ligands play very important role in
coordination chemistry and their metal complexes have
significant importance. It should be noted that these complexes
possess a number of important properties such as, easy
catalytic and biological properties have been studied
intensively.7 It is worth pointing out that complexation of the
ligands with various metals ions results an increasing biological
activity.8
The antibacterial and antifungal activities of different
transition metal complexes (Mn(II), Co(II), Ni(II), Cu(II) and
Pd(II)) with Schiff bases ligands have been reported.9 On the
basis of the fact that there is structural and thermodynamic
analogy between platinum(II) and palladium(II) complexes, the
study of anticancer activity of palladium(II) complexes is of
considerable importance. In addition, platinum based
compounds had not entered clinical trial for more than a
decade, which influenced development in research of other
metal compounds.10 A cytotoxic activities of different Schiff
base Pd(II) complexes was evaluated on wide range of cancer
cell lines.11 However, the results related to the examination of
anticancer activity of palladium(II) complexes with Schiff
bases as ligands are limited. One of the possible causes may be
the fact that many of them hydrolyse very fast, and that reaction
produces reactive compounds unable to act as potential
drugs.10,12
This journal is © The Royal Society of Chemistry 2013
J. Name., 2013, 00, 1-3 | 1

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
RSC Advances
Page 2 of 49
ARTICLE
VRiewSCArtAicldevOanlnincees
DOI: 10.1039/C5RA10204A
In the further course of our investigations of anil Schiff bases,13
All complexes exhibit nearly ideal square planar
coordination, with angles around palladium close to 90° (Fig. 2
and Tables S2ꢀS5). Each ligand (one nitrogen and one oxygen
donating atom) forms sixꢀmembered ring with palladium. It is
worth pointing out that the optimized parameters (bond
distances, angles, and dihedral angles) for Pdꢀ1 are in
agreement with reported values for Xꢀray structure.14
The NBO analysis revealed that, in all cases, there are no
covalent bonds of palladium with ligating atoms. Instead, there
is strong donation of electron density from the donor atoms to
palladium. Particularly, nitrogens delocalized their lone pairs
from the sp3 orbitals to the formally empty d orbital of
palladium(II), while oxygens contribute with lone pairs from
pure p orbitals. As a consequence, occupancies in the orbitals of
the donor atoms are reduced (1.62 and 1.67 respectively), while
the occupancy of palladium(II) formally empty d orbital is
increased (0.99).
our advanced step was to evaluate their cytotoxic activity and
oxidative stress status. Furthermore, we used four of them as N,Oꢀ
bidentate ligands to synthetize stable Pd(II) complexes, and to test
them for their cytotoxic activity and oxidative stress status, also. For
this purpose we used human adherent colorectal cancer cell line
(HCTꢀ116), human metastatic mammary gland breast carcinoma cell
line (MDAꢀMBꢀ231) and human fibroblast healthy cell line (MRCꢀ
5). Moreover, we used cisplatin (CisPt) as positive control, while
untreated cells were considered as negative control.
Results and discussion
In our previous work we presented antioxidative activity of
some salicylaldehyde and vanillic anil Schiff bases.13 In
addition to this, we synthesized four palladium(II) complexes,
Next, we wanted to confirm that suggested structures
correspond to the experimentally obtained complexes. For this
purpose we applied IR and NMR spectroscopy, as well as
Density Functional Theory. Namely, experimental data (IR and
13C NMR spectra) are compared to those theoretically obtained
for trans and cis isomers. It is worth pointing out that in all
cases trans isomers are more stable than the cis ones, Table 2.
starting from Nꢀsalicylidene aniline Schiff bases (1, 3, 5, and 6
from reference 13,) and palladium(II) acetate (molar ratio 2:1).
Our efforts to synthetize pure chelate complexes with the Schiff
bases 2, 4, and 7 were unsuccessful. However, the Schiff bases
examined in this paper are presented on Fig. 1. The structure of
the prepared complexes was elucidated using experimental and
theoretical tools. Biological activity of these complexes and of
their precursors was examined. It is worth pointing out that
Table 2. Difference in free energy (kJ/mol) of the corresponding cis and trans
complexes
some structural characterizations of investigated complexes can
14
be found in literature,
except for Pdꢀ3 which structural
ꢀGg
ꢀGsolvent
characterization was given first time now. Nevertheless, to our
best knowledge, this kind of characterization for investigated
complexes has not been reported until now.
(kJ/mol)
(kJ/mol)
Pdꢀ1
Pdꢀ3
Pdꢀ5
Pdꢀ6
14.56102
9.646087
16.75857
16.5249
5.849614
4.891306
10.73304
5.608068
On the basis of these facts, and experimental data (ref. 14), one
can undoubtedly conclude that obtained complexes are trans
isomers.
IR spectral characterization
At first glance, good agreement between experimental and
calculated spectra is achieved, Fig. 3. In all calculated spectra,
deviations from the experimental values are observed in the
region above 3000 cmꢀ1. OH stretching vibrations are
underestimated in case of ligands, while in the spectra of the
complexes (where OH group is still present), these bands are
overestimated. This can be attributed to the negligence of the
intermolecular forces present in the solid state. Nevertheless,
the calculated spectra of Schiff base ligands and corresponding
palladium complexes reveal the difference in their structure.
Namely, in the spectra of complexes Pdꢀ1 and Pdꢀ6 bands
assigned to OH stretching vibrations are somewhat changed,
while in cases of complexes Pdꢀ3 and Pdꢀ5, these bands are
completely absent from the spectra. This fact clearly shows that
the salicylaldehyde originating oxygen (from the deprotonated
phenolic group) became coordinated to palladium.
Fig. 1. The Schiff bases examined
Structural characterization of the investigated complexes
The optimized trans and cis geometries of investigated
complexes (Pdꢀ1, Pdꢀ3, Pdꢀ5, and Pdꢀ6) are presented in Fig. 2
and Fig. S1. Experimental and simulated IR spectra of
palladium(II) complexes and of their Schiff base precursors are
depicted in Figs.
3
and S2, while 13C NMR spectral
characterization is presented in Table 1 and Table S1. Bond
lengths, angles, and dihedral angles of all complexes calculated
are listed in Tables S1ꢀS4, while corresponding atom labelling
is depicted in Fig. S3.
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Page 3 of 49
Journal Name
RSC Advances
View Article Online
DOI: 10.1039/C5RAA10R2T04ICALE
predicted the 13C NMR spectra with high accuracy (AAE
amount to 2 ꢀ 5, and R above 0.97), also.
Comparison of the chemical shifts in the Schiff bases and
corresponding complexes (trans and cis) revealed that values
for carbon atom from azomethine group, for the one from
phenyl ring substituted with oxygen (salicylaldehyde
originating oxygen), as well as for the carbon from phenyl ring
bonded to nitrogen (aniline originating), are slightly elevated to
higher values (Table 1). In accordance to these are predicted
chemical shifts values. Similarly to the case of IR
characterization, this clearly points out that the nitrogen from
azomethine group and the salicylaldehyde originating oxygen
became coordinated to palladium. It is worth pointing out that,
as a consequence of Schiff bases coordination to the palladium,
chemical shifts for all aromatic carbons are to some extent
elevated, in both, experimental and theoretical spectra.
Similarly to the case of IR spectral characterisation, on the
basis of the obtained data, one can observe significant similarity
between 13C NMR spectra of the cis isomers and corresponding
trans isomers.
Fig. 2 The optimized geometries of trans complexes investigated.
In the spectra of ligands the bands in region 1615ꢀ1620 cmꢀ1
and 1620ꢀ1630 cmꢀ1 (experimental and calculated values
respectively) are assigned to the C=N stretching vibrations. In
the spectra of the corresponding complexes (trans and cis),
these vibrations are shifted to somewhat lower frequencies:
1600ꢀ1610 cmꢀ1 and 1590ꢀ1600 cmꢀ1, respectively. These shifts
are obviously a consequence of the formation of the palladium
complexes, and are in agreement with the NBO analysis, which
revealed donation of nitrogen`s lone pair to the formally empty
d orbital of palladium. Furthermore, comparison of the
experimental spectra of ligands and analogous spectra of the
complexes in the region of 505ꢀ540 cmꢀ1 and 460ꢀ470 cmꢀ1
showed that two new bands appeared in each spectrum of the
complexes. These new bands are assigned to newly established
PdꢀO and PdꢀN coordinative bonds, respectively. In addition,
inspection of the calculated spectra confirmed this assumption,
with the difference that these bands are slightly overestimated
(525ꢀ560 cmꢀ1 and 490ꢀ525 cmꢀ1). It is worth pointing out that
on the basis of theoretical results for cis isomers (Fig. S3) of
investigated complexes, one can conclude that their IR spectra
are very similar to the IR spectra of the trans isomers.
Biological evaluation
CYTOTOXIC EFFECTS
The cytotoxicity of investigated substances was determined by MTT
assay. The cytotoxic effects expressed as IC50 values for Schiff bases
and their corresponding Pd(II) complexes on HCTꢀ116 and MDAꢀ
MBꢀ231 cancer cell lines and on human fibroblast healthy MRCꢀ5
cell line are depicted in Table 3. It was shown that Pdꢀ1 and Pdꢀ6
show significant cytotoxic effects on all three cell lines, with IC50
values in range of 0.6 to 17.1 µM on HCTꢀ116 cells, 7.2 to 55.6 µM
on MDAꢀMBꢀ231 cells (with exception of Pdꢀ1 after 24 h from
treatment, IC50 = 276.9 µM) and 34.5 to 48.1 6 µM on MRCꢀ5 cells.
Bearing in mind that IC50 values obtained with CisPt on same cell
lines are in range of 26.9ꢀ >500 µM, results obtained with Pdꢀ1 and
Pdꢀ6 are very promising. Also, Pd(OAc)2, Pdꢀ3, and 1 exerted
higher cytotoxic effect in comparison to other Schiff bases and their
complexes. One should take in consideration great influence of
palladium(II) on cytotoxicity, especially in Pdꢀ1 and Pdꢀ6
complexes, whose ligands 1 and 6 do not exert such a significant
cytotoxic effects. Investigation of Pd(OAc)2, Pdꢀ3 and Pdꢀ5
complexes suggests that effects of investigated Pd(II) complexes
primarily depend on chemical structure, rather than on possible
hydrolyses of complexes. This assumption is supported by fact that
IC50 values for Pd(OAc)2, Pdꢀ3 and Pdꢀ5 are much higher than IC50
values of Pdꢀ1 and Pdꢀ6. Hydrolysed complexes should possess
similar cytotoxic activity as Pd(OAc)2, which under our measuring
condition was not recorded. Results presented in this work indicate
that HCTꢀ116 cells are more sensitive than MDAꢀMBꢀ231 cells,
which is in agreement with our earlier findings.15 It is important to
point out that significant difference in the sensitivity of the examined
cells arises from differences in origin of the tested cells. HCTꢀ116
cells are of primary tumour origin, while MDAꢀMBꢀ231 cells are
NMR SPECTRAL CHARACTERIZATION
The summary of 13C NMR data is presented in Table 1. The 13
C
NMR properties of the ligands and corresponding complexes
were predicted, and the chemical shifts for all carbon atoms
were calculated relative to TMS. On the basis of the
experimental and calculated shifts for the Schiff base ligands,
one can conclude that theoretical model reproduced
experimental NMR spectra with satisfactory accuracy. Namely,
the Absolute Average Errors (AAE) for 13C NMR amount to 2ꢀ
6 ppm. In addition, the correlation coefficients (R) for the
dependencies of the calculated chemical shifts on the
experimental values are larger than 0.95. Theoretical model
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 3