Synthesis, Characterization, and DNA-Binding Kinetics of New Pd(II) and Pt(II) Thiosemicarbazone Complexes: Spectral, Structural, and Anticancer Evaluation

Article abstract of DOI:10.1155/2020/3863269

In a bid to come up with potential anticancer agents, a class of thiosemicarbazone ligands bearing substituted thiophene were synthesized followed by complexation with various Pd(II) and Pt(II) metal precursors. The ligands (E)-1-((thiophen-2-yl)methylene)thiosemicarbazide (L1), (E)-1-((4-bromothiophen-2-yl)methylene)thiosemicarbazide (L2), and (E)-1-((5-bromothiophen-2-yl)methylene)thiosemicarbazide (L3) were synthesized by condensation reactions and obtained in good yields. Complexation of L1 and L2 with Pd(cod)Cl2 gave C1 (C6H7Cl2N3PdS2) and C2 (C6H6BrCl2N3PdS2), respectively. Complexation of L1 with K2PtCl4 gave C3 (C6H7Cl2N3PtS2), while L3 with K2PtCl2[(PPh)3]2 gave C4 (C24H21BrClN3PPtS2). The structures and coordination for all compounds were established by FTIR, 1H-NMR, 13C-NMR, UV-Vis, elemental analysis, and single-crystal X-ray diffraction studies for ligand L1. Tuning of the spectral and anticancer activity of the compounds was investigated by changing the position of the bromide substituent, metal center, and the σ or π-donor/acceptor strength of the groups surrounding the metal center. The compounds had low to moderate anticancer potency with their spectral and structural properties correlating with the corresponding anticancer activity profiles. DNA binding modes were studied by spectroscopy and were comparable to known DNA intercalators. Structure-activity profiles were evident especially between C1 and C2 differing by the presence of a Br in position 5 of thiophene ring, which caused a remarkable increase in IC50 values, from 14.71 ± 0.016 (C1) to 43.08 ± 0.001(C2) in Caco-2 cells, 1.973 ± 0.048 (C1) to 59.56 ± 0.010 (C2) in MCF-7 cells, 16.65 ± 0.051 (C1) to 72.25 ± 0.003 (C2) in HeLa cells, 14.64 ± 0.037 (C1) to 94.34 ± 0.003 (C2) in HepG2, and 14.05 ± 0.042 (C1) to >100(C2) in PC-3 cells.

Full text of DOI:10.1155/2020/3863269

Hindawi
Journal of Chemistry
Volume 2020, Article ID 3863269, 17 pages
https://doi.org/10.1155/2020/3863269
Research Article
Synthesis, Characterization, and DNA-Binding Kinetics of New
Pd(II) and Pt(II) Thiosemicarbazone Complexes: Spectral,
Structural, and Anticancer Evaluation
1
1
1
1
4
Simon N. Mbugua , Lydia W. Njenga, Ruth A. Odhiambo, Shem O. Wandiga,
2
2
3
Mervin Meyer , Nicole Sibuyi , Roger A. Lalancette, and Martin O. Onani
1
Department of Chemistry, University of Nairobi, P.O. Box 30197-00100, Nairobi, Kenya
Department of Biotechnology, University of the Western Cape, Private Bag X17, Belville 7535, South Africa
Department of Chemistry, Rutgers University, 73 Warren St., Newark, NJ 07102, USA
Department of Chemical Sciences, University of the Western Cape, Private Bag X17, Belville 7535, South Africa
2
3
4
Correspondence should be addressed to Simon N. Mbugua; mbuguasn@students.uonbi.ac.ke
Received 2 December 2019; Revised 22 April 2020; Accepted 22 May 2020; Published 27 June 2020
Academic Editor: Michele Benedetti
Copyright © 2020 Simon N. Mbugua et al. 'is is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is
properly cited.
In a bid to come up with potential anticancer agents, a class of thiosemicarbazone ligands bearing substituted thiophene were
synthesized followed by complexation with various Pd(II) and Pt(II) metal precursors. 'e ligands (E)-1-((thiophen-2-yl)
methylene)thiosemicarbazide (L1), (E)-1-((4-bromothiophen-2-yl)methylene)thiosemicarbazide (L2), and (E)-1-((5-bromo-
thiophen-2-yl)methylene)thiosemicarbazide (L3) were synthesized by condensation reactions and obtained in good yields.
Complexation of L1 and L2 with Pd(cod)Cl
of L1 with K PtCl gave C3 (C Cl PtS
2
gave C1 (C
6
H
7
Cl
2
N
3
PdS
2
2
) and C2 (C
6
H
6
BrCl
2
H
N
3
PdS
2
), respectively. Complexation
PPtS ). 'e structures and
2
4
6
H
7
2
N
3
2
), while L3 with K
2
PtCl [(PPh)
3
]
2
gave C4 (C24
21BrClN
3
2
1
13
coordination for all compounds were established by FTIR, H-NMR, C-NMR, UV-Vis, elemental analysis, and single-crystal
X-ray diffraction studies for ligand L1. Tuning of the spectral and anticancer activity of the compounds was investigated by
changing the position of the bromide substituent, metal center, and the σ or π-donor/acceptor strength of the groups surrounding
the metal center. 'e compounds had low to moderate anticancer potency with their spectral and structural properties correlating
with the corresponding anticancer activity profiles. DNA binding modes were studied by spectroscopy and were comparable to
known DNA intercalators. Structure-activity profiles were evident especially between C1 and C2 differing by the presence of a Br
in position 5 of thiophene ring, which caused a remarkable increase in IC50 values, from 14.71 ± 0.016 (C1) to 43.08 ± 0.001(C2) in
Caco-2 cells, 1.973 ± 0.048 (C1) to 59.56 ± 0.010 (C2) in MCF-7 cells, 16.65 ± 0.051 (C1) to 72.25 ± 0.003 (C2) in HeLa cells,
1
4.64 ± 0.037 (C1) to 94.34 ± 0.003 (C2) in HepG2, and 14.05 ± 0.042 (C1) to >100(C2) in PC-3 cells.
low- and middle-income countries have the necessary data
to drive cancer policy [1]. Although major steps have been
made in medicine, many difficulties still need to be re-
solved in order to boost cancer therapy. In line with this,
research is putting a great deal of effort into discovering
new and effective therapeutics that can mitigate the
pertinent adverse effects. Many research reports have
focused on creating alternative treatments to minimize the
side effects of traditional medicines over the past decade
[2–4].
1
. Introduction
Cancer is the second leading cause of death globally, with
an estimated 1 in 6 deaths, and is responsible for an es-
timated 9.6 million deaths in 2018 [1]. It is estimated that
approximately 70% of all deaths are cancer-related and
occur in low- and middle-income countries [1]. 'e
economic impact of cancer is significant and is increasing
with a total annual economic cost of cancer in 2010 es-
timated at approximately US$ 1.16 trillion. Only 1 in 5

2
Journal of Chemistry
−
1
'
e most practiced efforts to combat cancer include
4000–400 cm . Proton and carbon-13 nuclear magnetic
resonance spectra were obtained from the Bruker 400 MHz
spectrometer. 'e chemical shifts were measured in ppm
relative to a TMS standard. UV-Vis spectra were recorded in
DMSO solvent at a range of 200–800 nm. Bioassays on the
anticancer efficacy were performed at the Department of
Biotechnology at the University of the Western Cape. 'e
elemental microanalysis was performed on Server 112 series
Elemental Analyzer at the University of Stellenbosch. 'e
single crystal X-ray crystallography was performed at
Rutgers University, USA, using a Bruker SMART CCD
Apex-II area-detector.
surgery, chemotherapy using chemical drugs, and radio-
therapy using electromagnetic radiation like X-rays to kill
cancerous cells. New strategies like gene therapy are still
being developed where genes from donors are inserted into
the DNA of a patient to boost recognition of cancerous cells
by the body’s diseases fighting mechanisms. Some of the
most successful chemotherapeutic drugs include those de-
rived from metals like platinum, ruthenium, titanium, and
palladium, among others.
Research on thiosemicarbazone compounds for
medical applications appeared as early as 1950s when they
were used against TB and leprosy [5]. 'e following
decade saw these compounds tested for antiviral prop-
erties which were confirmed, leading to intense research
which eventually culminated to the commercialization of
some thiosemicarbazone products under trade names
'
e complexes (C1–C4) were tested against human cell
lines: Caco-2 (human colon), HeLa (human cervical),
HepG2 (human hepatocellular), MCF-7 (breast cancer),
PC-3 (human prostate), and MCF-12A (noncancer human
breast) cells. 'e cells were cultured in their respective
media supplemented with 10% fetal bovine serum (FBS)
and 1% pen-strep cocktail (100 U/mL penicillin and 100 g/
mL streptomycin). Caco-2, HeLa, HepG2, and MCF-7 cells
were grown in Dulbecco’s Modified Eagle Medium and
MCF-12A in DMEM-F12 containing insulin, EGF, and
hydrocortisone, while PC-3 cells were cultured in RPMI-
methisazone and Marboran , in the treatment of small-
®
pox [6]. Another major step in utilization of these
compounds was the publication of the first antitumor tests
[
7]. Finch and coworkers explored the broad areas of
application where thiosemicarbazones have received most
attention, classifying them in relation to their activities
against cancerous cells, protozoa, bacteria, or viruses.
°
1
640 media. 'e cells were grown at 37 C in a 5% CO
2
'
eir activity is found to correlate to the type of metal
humidified incubator (Labotec, South Africa) and seeded at
5
center [8]. As anticancer agents, the activity is shown to
substantially increase for the complexed product as
compared to the free ligand, and also, the undesirable
cytotoxicity as side effects was shown to be minimized in
the complexes [8].
1
× 10 cells/mL density in a 96-well plate for 24 h. 'e cells
were treated with 100 µg/mL of the test compound for 24 h
period. Cytotoxicity of the compounds was assessed by MTT
assay following manufacturer’s instructions. After treatment,
1
0 μL of 5 mg/mL MTT (Sigma, USA) was added to each well,
'
is work reports the anticancer activity of newly syn-
°
and the plates were incubated at 37 C for 3 h. 'e insoluble
thesized Pd(II) and Pt(II) complexes bearing thio-
semicarbazone ligands. 'e main objectives were to evaluate
the tuning of structure-activity relationships vis- a` -vis dif-
ferent σ and/or π-donor/acceptor properties of the groups
attached to the metal center, their anticancer activities, and
their binding modes to DNA.
formazan crystals were solubilised by adding 100 μL of DMSO.
'
e reduction of MTT was read at 570 nm using POLARStar
Omega plate reader.
2
.1. Synthesis of the Ligands
Anticancer and apoptotic properties were performed in
vitro using human colon (Caco-2), human cervical (HeLa),
human hepatocellular carcinoma (HepG2), breast cancer
2.1.1. (E)-1-((ꢀiophen-2-yl)methylene)thiosemicarbazide (L1).
'e ligand was synthesized using the method described by
Channar et al. [9] with some modifications. 'iophene-2-
carbaldehyde (475 µl (5 mmol)) and thiosemicarbazide
(0.4558 g (5 mmol)) were refluxed under inert conditions in
20 ml dry methanol for 12 hours. On cooling, a light brown
precipitate was formed which was redissolved in diethyl
ether, filtered on activated carbon, and kept in the fridge for
slow evaporation where shiny brown crystals were obtained.
(
MCF-7), human prostate (PC-3), and noncancer human
breast (MCF-12A) cells. Binding modes to DNA were
studied by use of calf thymus DNA (CT-DNA). A crystal
structure of one of the ligands, L1, is also reported here for
the first time according to the CCCD database.
2
. Materials and Methods
−
1
Yield: 54%. FTIR (KBr ] ); (3414–3149 N-H stretch)
2
All synthetic manipulations were performed under the fume
hood under inert conditions. 'e solvents were of analytical
grade, and each was dried with the recommended drying
agent and distilled prior to use. All reagents used in the
syntheses were of analytical grade procured from Sigma-
Aldrich or Merk Chemicals and used without further
modifications.
3000 (sp C-H bend), 1611 CH � N stretch, and 1538 C � S
stretch.
1
H-NMR (400 MHz, DMSO-d ) δ 11.45 (s, 1H),
6
8.09–8.33 (m, 1H), 7.65 (d, J � 5.01 Hz, 1H), 7.45 (dd,
J � 0.73, 3.55 Hz, 1H), 7.12 (dd, J � 3.67, 5.01 Hz, 1H), and
3.35 (s, 2H).
1
3
C-NMR (400 MHz, DMSO-d6) δ 178.27 (C � N im-
'
e melting points were determined on open capillaries
ine), 139.24, 138.22, 130.92, 129.33, and 128.41.
using SMP-10 melting point apparatus. FTIR spectra were
recorded using PerkinElmer Spectrum 100 Series ATR or
FT-IR instrument on KBr pellets in the range
Molecular weight: 185.27. Elemental analysis; molecular
formula; C H N S . Calculated (obtained); C, 38.90(38.83);
6
7 3 2
H, 3.81(3.87); N, 22.68(22.63); and S, 34.61(34.63).

Journal of Chemistry
.1.2. (E)-1-((5-Bromothiophen-2-yl)methylene)thiosemicar
bazide (L2). 'e ligand was synthetized with a synthetic
procedure similar for L1 using 4-bromothiophene-2-car-
baldehyde (625 µl (5 mmol)) and thiosemicarbazide
3
2
2.2.2. Synthesis of Pd(II) Complex (C2). 'e complex was
synthesized with a procedure similar to C1 using L2
(66 mg(0.25 mmol)). 'e complex was isolated as a brown
powder.
−
1
(
0.4558 g (5 mmol)). 'e ligand was isolated as a light brown
Yield 74%. FTIR (KBr ] ); 3075, NH stretch; 2943 CH
1
precipitate.
stretch; 1621 C � N; and 1419 C � S stretch. H-NMR
−
1
Yield 68%. FTIR (KBr ] ); (3429–3276 N-H stretch),
(400 MHz, DMSO-d ) δ 8.34 (s, 1H), 7.59 (s, 1H), 7.32 (s,
6
2
13
3
155–2999 (sp C-H bend), 1608 CH � N stretch, and 1543
1H), 6.97 (s, 1H), 5.76 (s, 1H), and 3.60 (br. s., 2H). C-
1
C � S stretch. H-NMR (400 MHz, DMSO-d ) δ 11.50 (s,
NMR (400 MHz, DMSO-d ) δ 172.83 (C � N imine), 144.86,
6
6
1
2
H), 8.14 (s, 1H), 7.64 (br. s., 1H), 7.24 (s, 1H), and 3.35 (s,
H)1.
136.01, 134.09, 130.64, and 124.92.
Molecular weight: 441.49. Elemental analysis; molecular
formula; C H BrCl N PdS . Calculated (obtained); C, 16.32
3
C-NMR (400 MHz, DMSO-d6) δ 184.27(C � N imine),
6
6
2
3
2
1
78.25 141.07, 137.29, 131.58, and 115.03.
(16.36); H, 1.37 (1.31); N, 9.52 (9.45); and S, 14.53 (14.57).
Molecular weight: 387.2. Elemental analysis; molecular
formula; C H ClN PdS. Calculated (obtained); C, 37.22
Molecular weight: 264.17. Elemental analysis; molecular
formula; C H BrN S . Calculated (obtained); C, 27.28(27.26);
H, 2.29(2.27); N, 15.91(15.96); and S, 24.28(25.27).
6
6
3 2
12 13
4
(37.27); H, 3.38 (2.33); N, 14.47 (14.42); and S, 8.28 (8.27).
2
.1.3. (E)-1-((4-Bromothiophen-2-yl)methylene)thiosemicar
2.3. Synthesis of Pt(II) Complexes
bazide (L3). 'e ligand was synthetized with a synthetic
procedure similar to L1 using 4-bromothiophene-2-car-
baldehyde (1.061 g (5 mmol)) and thiosemicarbazide
2
.3.1. Synthesis of Pt(II) Complex (C3). 'e complex was
synthesized by dropwise addition of a solution of K PtCl
2
4
(104 mg (0.25 mmol)) dissolved in MeOH/H O (2 :1) (3 ml),
(
0.4558 g (5 mmol)). 'e ligand was isolated as a light brown
2
into a stirring solution of L1 (46 mg (0.25 mmol)) in THF
powder.
−
1
(2 ml). A yellow precipitate was formed immediately, and
stirring was continued for one and half hours at room
temperature, filtered followed by washing with diethyl ether
Yield 58%. FTIR (KBr ] ); (3378–3246 N-H stretch),
2
3
155–3008 (sp C-H bend), 1605 CH � N stretch, and 1545
1
C � S stretch. H-NMR (400 MHz, DMSO-d ) δ 11.54 (s,
6
(
3 × 5 ml) and dried under vacuum.
1
H), 8.16 (s, 1H), 7.75 (s, 1H), 7.53 (s, 1H), and 3.34 (s, 2H).
−1
1
3
Yield 98%. FTIR (KBr ] ); 3268, NH stretch; 3082, CH
1
C-NMR (400 MHz, DMSO-d ) δ 178.27 (C � N imine),
6
stretch; 1593, C � N; 1515, C � S stretch. H-NMR (400 MHz,
DMSO-d ) δ 12.34 (br. s., 1H), 8.55 (s, 1H), 7.60 (br. s., 1H),
1
40.80, 136.35, 132.21, 126.46, and 109.97.
Molecular weight: 264.17. Elemental analysis; molec-
6
13
7
.16 (br. s., 1H), 6.95 (s, 1H), and 3.46 (br. s., 2H). C-NMR
ular formula; C H BrN S . Calculated (obtained); C, 27.28
6
6
3 2
(
400 MHz, DMSO-d ) δ 173.45 (C � N imine), 145.86,
(
(
27.22); H, 2.29 (2.24); N, 15.91 (15.87); and S, 24.28
24.27).
6
1
43.94, 136.93, 133.14, and 127.79.
Molecular weight: 451.25. Elemental analysis; molecular
formula; C H Cl N PtS . Calculated (obtained); C, 15.97
6
7
2
3
2
(15.91); H, 1.56 (1.51); N, 9.31 (9.38); and S, 14.21 (14.26).
2
2
.2. Synthesis of Pd(II) Complexes
.2.1. Synthesis of Pd(II) Complex (C1). 'e ligand L1 92 mg
0.25 mmol) was dissolved in THF (2 ml) and was added
(
2.3.2. Synthesis of Pt(II) Complex (C4). 'e complex was
synthesized following the procedure for the previous one by
adding of PtCl [(PPh ) ] (198 mg(0.25 mmol)) in DCM
dropwise into a stirring solution of Pd(cod)Cl2 (72 mg
0.25 mmol)) dissolved in DCM (15 ml) under argon. A red
(
2
3 2 2
precipitate was formed, and stirring was continued at room
temperature for 6 hours while monitoring the reaction with
TLC until completion. 'e solvent was reduced under
pressure to about 1 ml. Excess hexane was added followed by
filtration and washing of the red powder product with more
hexane (3 × 5 ml) followed by filtration and drying under
vacuum.
(5 ml) into a stirring solution of L2 (66 mg (0.25 mmol)) in
benzene (20 ml) at room temperature and stirred for 24 h.
'e complex was isolated as a yellow powder.
−
1
Yield 36%. FTIR (KBr ] ); (3287–3166 N-H stretch),
2
3076–2958 (sp C-H bend), 1608 CH � N stretch, 1582 C � S
1
stretch. H-NMR (400 MHz, DMSO-d ) δ 8.72 (s, 1H), 7.96
6
(s, 1H), 7.43 (m, 15H), 3.43 (s, 1H), 2.89 (s, 1H), and 2.74 (s,
2H).
−
1
Yield 71%. FTIR (KBr ] ); 3370, NH stretch;
3
176–3076, CH stretch, 1619, C � N, 1542; C � S stretch.
Molecular weight: 756.98. Elemental analysis; molecular
formula; C H BrClN PPtS . Calculated (obtained); C,
1
H-NMR (400 MHz, DMSO-d ) δ 8.35–8.60 (m, 1H), 7.98
6
24 21
3
2
(
s, 1H), 7.84 (s, 1H), 7.51 (br. s., 1H), 7.13 (d, J � 4.89 Hz,
38.08 (38.0); H, 2.80 (2.79); N, 5.55 (5.53); and S, 8.90 (8.87).
3
1
1
1
H), and 3.60 (s, 2H). 1 C-NMR (400 MHz, DMSO-d ) δ
6
77.24 (C � N imine), 172.16, 146.15, 135.36, 133.53, and
27.13.
2.4. Crystal Structure for L1. 'e single crystal X-ray crys-
tallography experiment was performed using a Bruker
SMART CCD Apex-II area-detector. 'e crystal structure
was solved and refined using the full-matrix least-square
Molecular weight: 362.6. Elemental analysis; molecular
formula; C H Cl N PdS . Calculated (obtained); C, 19.87
6
7
2
3
2
(19.77); H, 1.95 (1.96); and N, 11.59 (11.52).

4
Journal of Chemistry
2
method based on F [10] and the graphics interface program
PdCl and PtCl salts, respectively. As mentioned earlier,
2
2
ORTEP-3 for Windows [11].
complexation with Pd(II) caused a shift to higher energy by
−
1
−1
−1
8
cm (1611 cm to 1619 cm for C1) while complexation
with Pt(II) shifted the energy to a lower wavenumber by
8 cm (1593 cm to 1611 cm for C3). 'is can be
2
.5. Anticancer Activity. 'is was performed using human
−1
−1
−1
1
colon (Caco-2), human cervical (HeLa), human hepato-
cellular carcinoma (HepG2), breast cancer (MCF-7), human
prostate (PC-3), and noncancer human breast (MCF-12A)
cancer cells in vitro by incubating the particular cancer cells
explained in terms of nuclear charge for the two metals.
Pt(II) has a higher nuclear charge inferring that the in-
coming ligand will be attracted more to Pt than to Pd. 'is
leaves the azomethine region with less electron density and
more single bond character for the Pt(II) complex compared
to the Pd(II) complex. Bond dissociation energies for M-L
bond is also greater for Pt than Pd due to less steric crowding
around the Pt metal center. 'ese factors raise the overall
reactivity of Pd(II) complexes compared to Pt(II), a reason
−
4
with 2.2 ×10 M of the various compounds. Cell viability
was determined after a 24 h period.
Attempts to complex L2 with K PtCl and L3 with either
2
4
Pd(cod)Cl or K PtCl did not give a product. Similar at-
2
2
4
tempts to complex L1 with PtCl [(PPh ) ] did not yield a
2
3 2 2
product.
4
5
why Pd-Cl bond dissociates 10 -10 times faster in Pd(II)
complexes [12].
3
. Results and Discussion
Complex C4 was a modification of C3, where one -Cl
was substituted for -PPh and a -Br introduced at position 4
3
3
.1. Synthesis. 'e ligands L1–L3 were synthesized via Schiff
on the thiophene ring. 'e -PPh group is a much better s
3
base condensation reaction of thiosemicarbazide compound
with an appropriate thiophene-2-carbaldehyde under reflux
for 12 hours. 'e complexes were then formed through a
donor and π acceptor which effectively weakens the Pt-N
bond trans to itself in C4 through ground state trans-in-
fluence. 'is in turn means that electron density is not
drawn from the imine region, and therefore, the imine bond
in C4 retains more π character compared to that in C3.
However, the -Br in position 4 has minimal influence since
there is no resonance donation for the nonbonding elec-
trons. 'ese factors resulted in the observed difference in
reaction of the ligands with metal precursors Pd(cod)Cl (C1
2
and C2), K PtCl (C3), and PdCl (PPh ) (C4). 'ese re-
2
4
2
3 2
actions are summarized under Scheme 1.
3
.2. Infrared Spectra. 'e FTIR for the compounds were
recorded in either KBr pellets or ATR. 'e IR spectrum for
the ligands displayed a clear stretching vibration bands
between 1605 and 1611 cm which were assigned to the
imine C � N moiety. 'e ligand L1 has an unsubstituted
thiophene and shows the highest energy for the imine bond
at 1611 cm . On introducing a bromide at position 4 on the
thiophene ring (L3), there is a hypsochromic shift of 6 cm
−
1
−1
energy for the imine bonds, 1608 cm versus 1593 cm for
C4 and C3, respectively.
−
1
Complex C2 was a modification of C1, where a -Br was
introduced at position 5 on the thiophene ring. At position 5,
there is a possible resonance donation from the halide lone
pair implying that the imine bond retains more π character
in C4 compared to C1, reflected in the hypsochromic shift
−
1
−
1
−
1
−1
−1
to 1605 cm . 'is is because of the electron withdrawing
effect of the electronegative bromide which draws electron
density away from the thiophene ring and consequently
from the imine region thereby raising the single bond
character of the imine bond resulting in the observed lower
shift in the frequency.
observed in C2 (1621 cm from 1608 cm (L2)). 'e FTIR
spectra data for the complexes are shown in Table 1.
1
3.2.1. H-NMR Spectra. 'e azomethine protons together
with the protons attached to the thiophene ring were ob-
served and compared for the three ligands after introduction
of the bromide substituent, as shown in Figure 1.
'
e ligand L2 with a -Br introduced at position 5 on the
thiophene ring showed a lesser shift in frequency (3 cm )
−
1
−
1
−1
from 1611 cm to 1608 cm . Compared to L3, the -Br at
position 5 has a possible resonance donation from its lone
pair of nonbonding electrons which contribute to an in-
creased electron density to the thiophene ring and conse-
quently to the imine region. However, the electronegativity
dipole is dominant compared to the resonance dipole due to
the poor overlap of 4p-2p orbitals with carbon. 'e FTIR
spectra for the ligands are shown in Table 1.
For L1, H , H , and H occurred at 7.13, 7.45, and
a b c
7.65 ppm, respectively, while the azomethine proton oc-
curred at 8.25 ppm. Upon introduction of a Br at position 4
(L3), the remaining protons H and H appeared further
a c
downfield (7.53 and 7.75 ppm, respectively), as a result of
deshielding by the Br substituent. Similar trend is observed
when the Br is introduced at position 5 (L2). 'e peaks for
H and H now appear at 7.27 and 8.14 ppm, respectively.
a b
'
e azomethine peaks were observed in the complexes
Together with the electronegativity of the -Br, the protons at
orthoposition to -Br experience steric deshielding as the
hydrogen s-orbitals are disrupted from the normal spherical
symmetry.
where they were found to have shifted to between 1593 and
−
1
1
637 cm . It was observed that, in all the Pd complexes, the
azomethine peak shifted to higher energy, while for Pt(II)
complexes, it shifted to lower energy in the IR spectra. For
the ligands, a possible explanation to this can be deduced
from a consideration of the different groups and substituents
attached to the imine bond. Complexes C1 and C3 were
compared to the parent ligand L1 after the introduction of
'e azomethine proton for L1 appeared at 8.25 ppm. On
introduction of a Br at position 4 (L3) and position 5 (L2) of
the thiophene ring, an upfield shift of 0.10 ppm (to 8.16 ppm,
L3) and 0.11 ppm (to 8.14 ppm, L2) was observed. 'is trend
is expected because, at position 5, there is resonance

Journal of Chemistry
5
H
N
S
S
H
N
NH2
X1
X1
X2
NH2
N
O
Methanol
H N
2
S
+
Reflux, 12h
S
X2
X = H, X = H; L1
1
2
X = Br, X = H; L2
1
2
X = H, X = Br; L3
1
2
H
L1
N
S
N
H N
2
S
Pd
Cl
C1
THF/DCM
r.t, 24h
Pd (cod)Cl2
Cl
H N
2
L2
NH
S
Br
N
S
Pd
Cl
Cl
C2
H N
2
K PtCl
2
4
NH
L1
CH OH/H O/THF
3
2
S
N
room temp., 1.5h
S
Pt
Cl
Cl
C3
S
H
N
N
K PtCl (PPh )
H N
Br
2
2
3 2
2
L3
Pt
S
CH OH/H O/THF
Cl
3
2
room temp., 24h
P
C4
Scheme 1: Synthesis of Pd (II) and Pt (II) complexes.
Table 1: Photophysical data for ligands (L1–L3) and complexes (C1–C4).
4
−1
−1
FTIR
1H-NMR
13C-NMR
Absorption (ε x 10 LMol cm )
max (nm)
Imine (C � N)
Imine (CH � N)
Imine (C � N)
λ
L1
L2
L3
C1
C2
C3
C4
1611
1608
1605
1619
1621
1593
1608
8.25
8.14
8.16
8.41
7.59
8.55
7.96
139.09
141.16
140.58
146.04
145.02
145.86
142.98
340 (0.53)
347 (0.64)
343 (0.61)
364, 382 (1.51, 1.34)
371, 390 (1.16, 1.09, 0.0557)
348, 363, 540 (1.30, 0.86, 0.0087)
348, 401, 555 (1.41, 1.31)
S
Br
S
NH2
S
NH2
Hc
NH
NH2
NH
Hc
NH
N
N
N
S
S
S
Hb
Hb
Br
Ha
Ha
Ha
(
a)
(b)
Figure 1: Ligands (a) L1, (b) L2, and (c) L3.
(c)

6
Journal of Chemistry
donation of Br lone pair electrons; thus, the azomethine
After complexation, the shifts for the imine carbon of
complexes C1 and C3 occurred further downfield
(146.04 ppm and 145.86 ppm, respectively) compared to the
parent ligand L1. 'is increase in shift by 6.95 ppm and
5.28 ppm is as a result of coordination to the metal as
electron density is pulled away from the azomethine bond
deshielding the carbon nucleus. 'is means that only a
smaller external magnetic field is necessary to bring the
nucleus into resonance, resulting in the observed higher
chemical shifts in 13C-NMR. Similar trend is observed for
complexes C4 and C2 compared to the parent ligands. A
representative 13C-NMR spectrum for complex C2 is shown
in Figure 5.
proton for L2 experiences a higher shielding effect and
1
resonates further upfield compared to L3. 'e H-NMR
1
spectra data are shown in Table 1. A representative H-NMR
spectrum for ligand L3 is shown in Figure 2.
Upon complexation, the resonance frequencies of the
azomethine protons were observed and compared to those
of the free ligands. Complexes C1 and C3 were compared to
the parent ligand L1, where a downfield shift was observed in
both complexes. 'e Pd(II) complex (C1) showed a lesser
shift of 0.16 ppm compared to 0.30 ppm observed in the
Pt(II) complex C3. As discussed above, this can be attributed
to the higher nuclear charge of Pt(II) compared to Pd(II),
which causes a higher deshielding of the azomethine region
in the Pt(II) complex leading to a more downfield shift for
1
the azomethine proton. A representative H-NMR spectrum
3.2.3. Electronic Spectra. UV-visible spectra for the com-
for complex C1 is shown in Figure 3.
pounds were recorded in DMSO at range 200–800 nm using
−
3
Complexes C3 and C4 differed in that there was a Br
substituent attached to the thiophene ring, and the PPh3
group coordinated to the metal center in C4, while no
substituent was present in C3, and coordination to the metal
was by Cl ligands. Considering C4 with a Br at position 4 and
quartz cells at a concentration of 1.0 ×10 M. 'e results for
the photophysical data are summarized in Table 1.
As shown in Table 1, a higher bathochromic shift (7 nm)
was observed in L2 after introducing Br at position 5 of the
thiophene ring than in L3 (3 nm) with the -Br at position 4.
An auxochrome at position 5, as opposed to position 4, has
better p-π conjugation with the rest of the π system from the
possible resonance donation of the lone pair on -Br. 'is
increased conjugation led to the observed hyperchromic
effect in L2.
a PPh directly coordinated to the metal center, the Br at
3
position 4 has minimal resonance influence to the complex,
but PPh is both a good s donor and a π acceptor. 'e s
3
donation is through a hybrid orbital containing a lone pair
located on phosphorus, which is enhanced by the electron-
rich phenyl rings attached to the phosphorus. 'e steric bulk
'e complexes had different metal centers and ligands
coordinated to the metal. C1 and C3 differed only by the
type of metal center. 'e absorbance for C3 with a Pt (II)
metal center occurred at a shorter wavelength (348 nm)
compared to C1 (364 nm) with a Pd(II) metal center. A
possible reason for this is the higher nuclear charge of Pt(II)
which draws electron density away from the π system ef-
fectively lowering the energy of π orbitals thereby increasing
of the PPh group and the resultant crowding around the
3
metal center is a determinant in the rate at which the group
dissociates from a metal, where the bulkier the R group, the
faster the rate of dissociation [13]. 'ese factors together
with the higher trans-influence of PPh compared to Cl
3
make C4 to be more shielded at the azomethine region
compared to C3. 'is resulted in the observed difference of
∗
0
.59 ppm between the azomethine protons of the two
the ΔE between π-π levels in C3. C1 and C2 differed only by
complexes.
Complexes C2 and C1 differed only on the Br substituent
the presence of a -Br in position 5 of C2. 'e presence of the
auxochrome increased the electron density of the π bonding
orbitals resulting in raised energy level through electron-
electron repulsion, leading to the observed longer wave-
length in C2.
at position 5 of the thiophene ring of C2. At position 5, the
Br substituent has possible resonance donation to the ad-
jacent π system. 'is increases the shielding of the azo-
methine proton making it to resonate at a higher field region
C4 and C3 differed in that there was a -Br substituent in
1
compared to that of C1. 'e H-NMR spectra for the
position 4 of the thiophene ring, and a bulky -PPh
3
coor-
dinated to the metal center of C4. PPh is a better s donor-π
3
complexes are shown in Table 1.
acceptor compared to Cl, and the electron-rich phenyl rings
impart it with better ligand to metal charge transfer prop-
erties causing the observed red shift in C4. 'e UV-Vis
spectra for the complexes are displayed in Table 1.
1
3
13
3
.2.2. C-NMR Spectra. 'e C-NMR spectra for the ligands
were compared with those of their corresponding complexes
for carbons at the imine regions. 'e ligand imine carbons
shifts occurred at 139.24 ppm, 141.07 ppm, and 140.80 ppm for
L1, L2, and L3, respectively. Introduction of a -Br at position 5
In general, ligand spectra showed peaks corresponding
∗
to π ⟶π transitions with molar extinction coefficients of
3
−1
−1
10 LMol cm , while the complexes exhibited an extra
broad and weak absorptions at around 550–570 nm assigned
to d-d transitions, with molar extinction coefficients of
(
L2) had an incremental effect on the chemical shift (by
1
.83 ppm) on the imine carbon compared to the -Br at position
1
2
−1
−1
4
(L3) which caused a shift of 1.56 ppm. At position 5, the -Br
10 –10 LMol cm . In contrast, the spectra for all the
complexes displayed two peaks for C1 and C2 and three
peaks for C3 and C4. Similar observations are reported in
literature and are attributed to a combination of the ligand
has a higher inductive effect on the ligand via the π system,
which deshields the imine carbon more causing it to resonate
13
at a higher frequency (low field). 'e C-NMR spectra for the
13
∗
∗
ligands are shown in Table 1. A representative C-NMR
spectrum for ligand L3 is shown in Figure 4.
based π-π and MLCT [(dπ
−π (ligand)] [14]. 'e UV-
(metal)
Vis spectra for complex C3 is shown in Figure 6.

Journal of Chemistry
7
1
S
7
5
NH
1
NH
3
2
8
6
N
4
2
S
9
10
1
1
Br
1
2
8
.4 8.2 8.0 7.8 7.6
ppm
8
5
1
0
3
16
15 14 13 12 11 10
9
8
7
6
5
4
3
2
1
0
–1 –2 –3
δ ppm
1
Figure 2: H-NMR spectra for ligand L3.
3
S
8
6
+
N
NH
1
2
9
7
N
2
4
1
0
11
S
–
Pd
5
1
2
Cl
Cl
1
3
1
4
1
6
15 14 13 12 11 10
9
8
7
6
5
4
3
2
1
0
–1 –2 –3
δ ppm
1
Figure 3: H-NMR spectra for complex C1.
3
.2.4. Crystal Structure for Ligand L1. Crystals suitable for
'e molecular pair adopts a different conformation for
each unit molecule with one molecule lying on a perfect
X-ray crystallography were grown for ligand L1 by slow
diffusion of hexane into a THF solution of L1. 'e ligand
belongs to the monoclinic P21/n space system with 8-unit
molecules packed into the crystal unit. 'e structure data
and refinement parameters are shown in Table 2. 'e
structure showed the thiosemicarbazone existing in the
thione form.
°
plane but the other having a torsion angle of about 17
(Figure 7(a)). 'e molecule is stabilized through intermo-
lecular hydrogen bonding between the hydrogen attached to
N-5 of one molecule and thiosemicarbazide sulphur of the
other molecules. Furthermore, hydrogen bonding extends
towards the adjacent molecular pair, as shown in Figure 7(b).

8
Journal of Chemistry
S
7
5
NH
3
NH
1
2
8
6
N
4
2
S
9
10
1
1
Br
2
1
2
10 200 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10
0
–10
δ ppm
Figure 4: 13C-NMR for L3.
S
5
NH
3
NH
1
Br
+
2
7
1
5
8
6
N
2
4
9
10
–
S
Pd
1
2
11
Cl
Cl
1
3
1
4
2
10 200 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10
0
–10
δ ppm
Figure 5: 13C-NMR for C2.
Proton scans over a 72 h period were conducted to es-
3.3. Bioassays on Cytotoxicity. 'e main objective of this
work was to evaluate thiosemicarbazone-based Pd(II) and
Pt(II) complexes for possible anticancer properties. 'e
motivation behind this study was two-fold. First, the ste-
rically bulky substituents around the metal center are a
strategy appearing in literature for prevention of axial ap-
tablish stability in solution, as shown in Figure 8. Lack of any
changes in peaks over the test period indicated that the
compounds were stable in solution and therefore could be
studied for bioassay.
'
e complexes were then subjected for bioassays to
2
assess their anticancer activities.
proach to the metal atom from the z -direction. 'is hinders

Journal of Chemistry
9
3
.5
2
2
1
0
.5
1
.5
0
2
32
332
432
532
632
732
–
0.5
Wavelength (nm)
Figure 6: UV-Vis spectra for complex C3.
Table 2: Crystal data for L1.
Chemical formula
6 7 3 2
C H N S
Mr
185.27
Crystal system, space group
Temperature (K)
Monoclinic, P21/n
100
˚
a, b, c (A)
13.4381 (2), 5.7728 (1), 21.2683 (4)
°
β ( )
96.302 (1)
1639.93 (5)
8
Cu Kα
5.37
3
˚
V (A )
Z
Radiation type
−
1
µ (mm )
Crystal size (mm)
0.37 × 0.23 × 0.19
Data collection
Diffractometer
Bruker SMART CCD Apex-II area-detector
T
min, Tmax
0.272, 0.496
21136, 2897, 2744
0.041
No. of measured, independent, and observed (I > 2σ(I)) reflections
R int
−
1
˚
(
sin θ/λ)max (A )
0.596
Refinement
2
2
2
R[F > 2σ(F )], wR(F ), S
0.026, 0.067, 1.04
No. of reflections
2897
200
No. of parameters
H-atom treatment
H-atom parameters constrained
0.37, −0.27
−3
˚
Δρmax, Δρmin (e A )
(
a)
(b)
Figure 7: (a) ORTEP view for the molecular pair for ligand L1 showing the torsion in one molecule; (b) the crystal packing and H-bonding.

10
Journal of Chemistry
1
1
2
1
0
1
9
8
7
6
5
4
3
2
1
16
15 14 13 12 11 10
9
8
7
6
5
4
3
2
1
0
–1 –2 –3
δ (ppm)
(
a)
1
1
2
1
0
1
9
8
7
6
5
4
3
2
1
16
15 14 13 12 11 10
9
8
7
6
5
4
3
2
1
0
–1 –2 –3
δ (ppm)
(
b)
Figure 8: Continued.

Journal of Chemistry
11
1
1
2
1
0
1
9
8
7
6
5
4
3
2
1
16
15 14 13 12 11 10
9
8
7
6
5
4
3
2
1
0 –1 –2 –3
δ (ppm)
(
c)
I
Figure 8: H-NMR scans for (a) C1, (b) C2, and (c) C4 at 6 h intervals for 72 h in the presence of D O.
2
the formation of a trigonal bipyramid (TBP) intermediate
which would lead to a ligand substitution [15] and thereby
lead to high binding selectivity to DNA in vitro and in vivo
of -Br in C2 which decreased the electron density around the
metal center. 'is caused the -Cl ligands to be held more firmly,
which in turn decreased the rate of -Cl substitution reducing the
cytotoxicity of C2. Similar observations are reported in literature
where it is observed that an increase in donor properties of the
coordinated groups directly leads to increased cytotoxicity for
Pd drugs [19].
Complexes C1 and C3 had similar structures differing
only by the metal center, Pd(II) for C1 and Pt(II) for C3.
Complex C1 was found to be more cytotoxic than C3 in all
the tested cells. Since Pd(II) has a lower nuclear charge
compared to Pt(II), the ligands are less strongly held in C1.
'e smaller spatial size of 4d-orbitals in Pd (II) compared to
[16]
Secondly, it has been shown that bidentate chelation
plays a significant role in preventing trans-labilization and
unwanted dislodgment of ligands in vivo [17, 18]. Prelim-
inary results with the free ligands did not show any sub-
stantial activity either as anticancer agents, and therefore, the
detected activity could be accredited to the complexed
forms.
'
e anticancer activity was calculated according to the
following equation:
5
d-orbitals in Pt (II) also led to more steric crowding in the
average absorbances of treated cells
average absorbances of control
Pd (II) centre which probably resulted in higher rates of
ligand substitution, raising the cytotoxicity in C1.
%
cell viability � ꢀ
ꢁ
'
e observed higher cytotoxicity in C4 in comparison
×
100%.
to C3 could be attributed to the steric crowding effects of
the bulky PPh group in C4 since the steric bulk of a PR
3
3
(
1)
group and the resultant crowding around the metal center
is a determinant in the rate at which the group dissociates
from the metal. Tolman [13] working on nickel complexes
made similar observations where steric hindrance played a
bigger role than electronic effects in influencing the rate of
attaining equilibrium in a number of phosphorus ligands
on nickel(0) [13]. 'e expected higher ligand dissociation
determined the observed higher cytotoxicity for C4
compared to C3 with a Cl group. 'e plain ligands did not
show any substantial activity at the tested dose (data not
shown) compared to their corresponding complexes, and
therefore, the reduced cell proliferation shown could be
accredited to the complex compounds.
'
e results obtained against a control under similar test
conditions with no test compound added are shown in
Figure 9.
Structure-activity correlations were drawn on the different
trends in cytotoxicity of these complexes. Complex C1 was
similar to C2 differing only by a -Br substituent on position 5 of
the thiophene moiety of C2. Comparing the results for these two
complexes, the introduction of the -Br (C2) resulted in re-
markable difference in activity, where cell viability in all cell lines
Caco-2 (8%), HepG2 (5%), HeLa (38%), MCF-7 (6%), MCF-
1
2A (5%), and PC-3 (49%) increased remarkably. 'e difference
in the results for C1 and C2 could be attributed to the presence

12
Journal of Chemistry
1
1
20
00
120
100
80
60
40
20
0
8
6
4
2
0
0
0
0
0
Control
C1
Control
C2
(
a)
(b)
1
1
20
00
120
100
80
60
40
20
0
8
6
4
2
0
0
0
0
0
Control
C3
Control
C4
(
c)
(d)
Figure 9: Percent cell viability for the different compounds (C1–C4) after 24 h of treatment assessed by the colorimetric MTTassay: (a) C1;
b) C2; (c) C3; (d) C4.
(
Table 3: 'e IC50 values for ligands and complexes C1–C4.
Complex (µM)
C1
C2
C3
C4
Ligands
Caco-2
MCF-7
HeLa
HepG2
PC-3
14.71 ± 0.016
43.08 ± 0.001
17.52 ± 0.005
31.14 ± 0.002
>100 (L1–L4)
1.973 ± 0.048
59.56 ± 0.010
12.52 ± 0.011
70.09 ± 0.006
>100 (L1–L4)
16.65 ± 0.051
72.25 ± 0.003
19.2 ± 0.003
46.25 ± 0.011
>100 (L1–L4)
14.64 ± 0.037
14.05 ± 0.042
94.34 ± 0.003
>100
29 ± 0.003
48.03 ± 0.006
12.63 ± 0.036
>100 (L1–L4)
>100 (L1–L4)
6.98 ± 0.008
'e IC50 values for the complexes are tabulated in
Table 3.
on the d-d transitions, as the DNA concentration is serially
increased to a solution of a metal complex. Binding modes are
then inferred from the spectra, whereby an intercalative mode
results in a hypochromic shift and bathochromic shift or
∗
3
.4. DNA-Binding Studies. Spectroscopy is widely used in the
hypsochromism. Intercalation is as a result of overlap of π
study of binding modes for metal complexes by observing the
antibonding orbitals of the ligands bound to the complex with
π bonding orbitals of the DNA base pairs. 'is decreases the
probable transitions, and the effect is observed as
∗
alterations that appear in the λmax absorption values of π-π
intraligand transitions, ligand⟶metal charge transfers, or

Journal of Chemistry
13
4
.5
3
3
2
2
0
5
0
3
2
1
0
15
y = 8E – 05x + 2.9625
R2 = 0.9955
1
0
5
0
.5
2
0
100000 200000 300000 400000
1
/[DNA]
.5
1
.5
0
2
32
282
332
382
Wavelength, nm
432
482
532
0
3
6
1
μM
26μM
48μM
84μM
.4μM
.8μM
4μM
Figure 10: UV-visible absorption spectra for the C1 at 18 µM without CT-DNA and with serial addition of DNA. Inset is the Bene-
si–Hildebrand plots for the determination of binding constant.
hypochromic response [20]. On the contrary, a hyperchromic
shift suggests electrostatic mode of binding. 'e spectral shift
in the metal complex absorption is correlated to the binding
strength from which the kinetics can then be determined [21].
Observations on the changes occurring in the UV-Vis
spectra of a compound during serial additions of DNA is an
accepted procedure in studying the binding modes and kinetics
of DNA-complex interactions [22]. In this study, we used calf
thymus-DNA (CT-DNA) prepared in 10 mM Tris-HCl buffer at
pH 7.29. 'e DNA concentration was calculated from the λmax
of 260 nm using the molar absorptivity of the CT-DNA and
AO
εG
εG
1
� ε
+ ε
,
(2)
A − AO
K [DNA]
H−G
H−G
b
where A
and A are the absorbances of the blank sample
o
without DNA and absorbance of complex DNA after ad-
dition of serial concentrations of DNA, respectively, while ε
G
and εH−G are the molar extinction coefficients of complex
only and complex DNA, respectively. 'e binding constant
is determined by plotting A
/(A − A
) vs 1/[DNA] to give a
o
o
straight line graph. 'e equation of the straight line gives the
slope and intercept where the K value was calculated from
b
−
1
ε � 6600 M . 'e ratio of the absorbance (A /A280) was found
the ratio of the intercept to the slope. 'e strength of the
interaction is reflected by the value of the binding constant.
From the binding constant values, Gibbs free energy
changes, ΔG, for the complex DNA interactions were cal-
culated using the following equation:
260
to be 1.918 which assured that DNA was free of protein [23].
Serial concentrations of calf thymus DNA (CT-DNA)
were titrated to a fixed concentration of the complexes in the
cuvette and scanned from 200–800 nm. 'e mixture was
allowed to incubate for 5 minutes prior to taking UV-Vis
measurements. With continued addition of DNA, the peaks
of interest eventually decreased up to a point where they
disappeared, and at the same time, there appeared a peak at
−1
ΔG � −RT ln K kJmol ,
(3)
b
where R is the gas constant and T is the absolute
temperature.
2
60 nm. 'is clearly showed that all the complex had been
'
e absorption spectra of the interaction of CT-DNA
bound to DNA and further additions resulted in excess free
DNA in the mixture which was indicated by the appearance
of a characteristic DNA peak at 260 nm.
with the complexes together with the Benesi–Hildebrand
plots are shown in Figures 10–13.
All the complexes exhibited hypochromic shifts at λ
.
max
'e binding kinetics for the complex-DNA interaction was
'
e hypochromism is probably the result of intercalative
∗
then determined by the intrinsic binding constant, K , by plots
of the Benesi–Hildebrand equation [24] and shown as follows:
b
mode of binding as a result of the overlap of π antibonding
orbitals of the ligands bound to the complex, with π bonding

14
Journal of Chemistry
3
2
1
0
.5
3
0
.00E + 00
5.00E + 04
1.00E + 05
1.50E + 05
0
–10
y = –0.0004x + 1.3612
2
–
–
–
–
20
30
40
50
R = 0.999
.5
2
.5
1
1
/[DNA]
.5
0
2
32
282
332
382
432
37.2μM
53.1μM
58μM
482
532
0
1
1
1
μM
0.1μM
3.4μM
9.7μM
Figure 11: UV-visible absorption spectra for the C2 at 18 µM without CT-DNA and with serial addition of DNA. Inset is the Bene-
si–Hildebrand plots for the determination of binding constant.
3
2
1
0
.5
3
0.00E + 00
2.00E + 05
4.00E + 05
0
–
–
10
20
y = –0.0002x – 3.4596
2
R = 0.9933
.5
2
–30
–
–
–
40
50
60
.5
1
1/[DNA]
.5
0
2
33
283
333
383
433
483
533
583
Wavelength, nm
0
3
6
1
μM
26μM
48μM
67.4μM
82.3μM
.4μM
.8μM
3μM
Figure 12: UV-visible absorption spectra for the C3 at 18 µM without CT-DNA and with serial addition of DNA. Inset is the Bene-
si–Hildebrand plots for the determination of binding constant.
orbitals of the DNA base pairs [25]. 'is decreases the
probable transitions, and the effect is observed as a hypo-
chromic shift [20].
Furthermore, distinct isosbestic points appeared at
283 nm in the spectra of C2 and C4 and 292 nm for C3.
'is is as a result of equilibrium between bound and free

Journal of Chemistry
15
4
.5
3
0
.00E + 00
5.00E + 04
1.00E + 05
1.50E + 05
2.00E + 05
0
3
2
1
0
–
50
y = –0.0017x + 32.235
–
–
–
–
100
150
200
250
2
R = 0.9019
.5
2
.5
1
1
/[DNA]
.5
0
2
40
290
340
390
Wavelength, nm
440
490
540
0
6
1
μM
19.7μM
37.3μM
53.1μM
.8μM
3.4μM
Figure 13: UV-visible absorption spectra for the C4 at 18 µM without CT-DNA and with serial addition of DNA. Inset is the Bene-
si–Hildebrand plots for the determination of binding constant.
Table 4: Comparison of binding constants (K ) and Gibbs free energy values for the complexes-DNA interactions derived from UV
b
spectrophotometric data at pH 7.21 with other reported intercalators.
−
1
−1
Complex
Mode of binding
Binding constant, K
b
(M )
Gibbs free energy, ΔG (kJ mol )
References
4
C1
Intercalative
Intercalative
Intercalative
Intercalative
Intercalative
Intercalative
Intercalative
Intercalative
3.7031 × 10
−26.03
−20.15
−24.18
−24.04
−35.06
−24.19
−25.85
−24.90
CW
CW
CW
CW
[27]
[28]
[29]
[29]
3
C2
3.403 ×10
4
C3
1.7298 ×10
4
C4
1.8962 ×10
6
Ethidium bromide
Lumazine
Epirubicin
Proflavine
1.4 ×10
4
1.74 ×10
4
3.4 ×10
4
2.32 ×10
CW � current work.
form of DNA and indicates that there are no other
species in the mixture [26]. Binding constant and Gibbs
free energy values were then evaluated and compared to
those of common DNA intercalators, as shown in
Table 4.
system. 'is may have contributed to the low binding profile
of C2.
As shown in Table 4, all the ΔG values were negative, an
indication of the spontaneity of interaction during the
complex-DNA formation process [28].
Complex C1 showed higher values compared to those of
the known DNA intercalators lumazine, epirubicin, and
proflavine, indicating a strong binding between these
complexes and DNA. Complex C3 showed a lower binding
constant probably due to the presence of the Br substituent.
Since intercalation generally arises from the overlap of
4
. Conclusion
'is work produced new complexes of Pd(II) and Pt(II)
with insights into tuning the physical-chemical properties
and the resultant biological effects. Synthesis, isolation, and
purification were all performed under mild laboratory
conditions. 'e suggested structures were confirmed by
elemental microanalysis and X-ray crystallography for one
of the ligands.
∗
π -orbitals of intercalator with the π-orbitals DNA base
pairs [30], the presence of an electronegative substituent
conjugated to the π-system of an intercalator will remove
electron density, thereby reducing the reactivity of the

16
Journal of Chemistry
Binding profiles with DNA were studied using CT-DNA
and UV-Vis spectroscopy, where they exhibited hypo-
chromic shifts at λ which indicated an intercalative
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