Synthesis, crystal structure and in vitro anticancer studies of bis(dibenzyldithiocarbamato)Zn(II)

Article abstract of DOI:10.1080/00958972.2021.1887482

Bis(dibenzyldithiocarbamato)Zn(II), [Zn(dbzdtc)₂], was synthesized and characterized by spectroscopic techniques, elemental analysis and single crystal X-ray crystallography. The compound is crystallized in a monoclinic space group P2₁/n with Zn(II) located on crystallographic twofold symmetry and coordinated to two molecules of dibenzyl dithiocarbamato anions to form a rare four-coordinate mononuclear Zn(II) complex in a see-saw geometry. The cytotoxicity of the complex was evaluated by MTT assay against breast cancer cells (MCF-7) and human embryonic kidney cells (HEK293). The results showed that the complex is active against MCF-7 breast cancer cells with IC₅₀ value of 6.41 and 4.423?μΜ against HEK293 cell line.

Full text of DOI:10.1080/00958972.2021.1887482

Journal of Coordination Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Synthesis, crystal structure and in vitro anticancer
studies of bis(dibenzyldithiocarbamato)Zn(II)
Peter A. Ajibade, Berlinda M. Sikakane, Abimbola E. Oluwalana, Athandwe
M. Paca & Moganavelli Singh
To cite this article: Peter A. Ajibade, Berlinda M. Sikakane, Abimbola E. Oluwalana, Athandwe
M. Paca & Moganavelli Singh (2021) Synthesis, crystal structure and inꢀvitro anticancer studies
of bis(dibenzyldithiocarbamato)Zn(II), Journal of Coordination Chemistry, 74:8, 1244-1254, DOI:
10.1080/00958972.2021.1887482
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JOURNAL OF COORDINATION CHEMISTRY
2021, VOL. 74, NO. 8, 1244–1254
https://doi.org/10.1080/00958972.2021.1887482
Synthesis, crystal structure and in vitro anticancer studies
of bis(dibenzyldithiocarbamato)Zn(II)
Peter A. Ajibadea , Berlinda M. Sikakanea, Abimbola E. Oluwalanaa,
Athandwe M. Pacaa and Moganavelli Singhb
b
aSchool of Chemistry and Physics, University of KwaZulu-Natal, Scottsville, South Africa; School of
Life Sciences, University of KwaZulu-Natal, Durban, South Africa
ABSTRACT
ARTICLE HISTORY
Received 26 May 2020
Accepted 8 December 2020
Bis(dibenzyldithiocarbamato)Zn(II), [Zn(dbzdtc)2], was synthesized
and characterized by spectroscopic techniques, elemental analysis
and single crystal X-ray crystallography. The compound is crystal-
lized in a monoclinic space group P21/n with Zn(II) located on
crystallographic twofold symmetry and coordinated to two mole-
cules of dibenzyl dithiocarbamato anions to form a rare four-
coordinate mononuclear Zn(II) complex in a see-saw geometry.
The cytotoxicity of the complex was evaluated by MTT assay
against breast cancer cells (MCF-7) and human embryonic kidney
cells (HEK293). The results showed that the complex is active
against MCF-7 breast cancer cells with IC50 value of 6.41 and
4.423 lM against HEK293 cell line.
KEYWORDS
Dibenzyl dithiocarbamate;
Zn(II) complex; crystal
structure; anticancer;
cytotoxicity
CONTACT Peter A. Ajibade
ajibadep@ukzn.ac.za
School of Chemistry and Physics, University of KwaZulu-
Natal, Pietermaritzburg Campus, Private Bag X01, Scottsville 3209, South Africa
Supplemental data for this article is available online at https://doi.org/10.1080/00958972.2021.1887482.
ß 2021 Informa UK Limited, trading as Taylor & Francis Group

JOURNAL OF COORDINATION CHEMISTRY
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1. Introduction
There is an urgent need to develop metal complexes as novel chemotherapeutic
agents as alternative to cisplatin and its derivatives due to their toxic side effects and
inherent resistance [1–3]. The evolution of therapeutic drugs using metals such as
zinc, iron, copper, palladium or ruthenium [4] is the main objective of bio-inorganic
medicinal chemistry research. Metal complexes of chelating ligands such as N,S-amino-
thioether [5], N-N-diamines [6], dicarboxylic acids [7], sulfonamide [8] and dithiocarba-
mate [9] are explored as potential anticancer agents. Among these compounds, sulfur
containing compounds are receiving attention due to their resemblance to biomole-
cules such as vitamins and amino acids, which enhances the transportation of the
drugs to the target cells [10]. Organosulfur ligands such as dithiocarbamate have
received attention due to the ability of the dithiocarbamates to coordinate metal ions
in different oxidation states [11]. Dithiocarbamate transition metal complexes of
copper(II), zinc(II), manganese(II), cobalt(II) and ruthenium(II) have been studied as
potential anticancer agents [12–16]. Zinc is one of the trace elements required for
DNA stabilization, cell proliferation, enzyme and immune system function due to its
low toxicity and excellent biocompatibility [17]. Thus, research on zinc(II) dithiocarba-
mate biological activity is of importance.
Shiri et al. [18] prepared 5-dithiocarbamato-1,3,4-thiadiazole-2-thiol and its Zn(II)
complex, which was screened for anticancer activity against MCF-7 cells. The results

1246
P. A. AJIBADE ET AL.
showed that the ligand had no noticeable cytotoxic effects when compared to the
complex, which suggests that cytotoxic effects are enhanced upon complexation with
the metal ion [19]. Singh et al. [19] screened a series of Zn(II) binuclear dithiocarba-
mate complexes against the hepatoma (HepG2) cell line. The screened compounds
had better activity compared to cisplatin and shrinkage of the cells was observed due
to apoptosis inducing ability of the compounds [19]. Maurya et al. [20] reported func-
tionalized dithiocarbamate Zn(II) complexes activity as antibacterial, antibiofilm and
antitumor agents; the compounds showed better antibacterial activity. Zn(II) com-
plexes have been reported to have antiproliferative effects on various cancer cells [16,
18] and tumorigenesis effect due to dosage concentration [19]. Also, Zn(II) complexes
have been used as photodynamic therapeutic agents [20] and carrier conjugates to
target cells [21]. Another interesting attribute of Zn(II) dithiocarbamate is the ability to
exist in diverse structural geometries [22]. Zinc(II) bis(dithiocarbamate) complexes are
mostly dimeric with distorted octahedral, tetrahedral, trigonal–bipyramidal or square–-
pyramidal geometry [10, 14, 19, 23–31]. Monomeric configuration is possible in rare
cases that are most often reported as distorted tetrahedral geometry [22, 32–34]. In
this study, we report the synthesis, spectroscopic characterization and crystal structure
of a mononuclear bis(dibenzyldithiocarbamato)Zn(II) complex in a see-saw geometry
and its cytotoxic evaluation against human cancer cell (MCF-7) line and human embry-
onic kidney cells (HEK293) using cisplatin as a standard.
2. Experimental
2.1. Materials and physical measurements
All chemicals and reagents were used as received. Dibenzylamine, sodium hydroxide,
ethanol, CS2, methanol and DMSO were obtained from Sigma-Aldrich (St. Louis, MO,
1
USA) and Merck (Darmstadt, Germany). H and 13C NMR spectra were recorded on 400
and 100 MHz Bruker NMR spectrometers, respectively. Elemental analysis (C, H, N and
S) was carried out using a Thermoscientific Flash 2000. Infrared spectra were obtained
with an Agilent Technologies Cary 630 FTIR spectrophotometer from 4000 to
600 cmꢀ1 using the KBr disk method. Mass spectra were recorded on a LC Premier
micro-mass spectrometer and UV–Vis spectra were recorded on a Perkin-Elmer
Lambda 25 UV–Vis spectrophotometer.
2.2. Synthesis of ligand and complex
2.2.1. Synthesis of dibenzyldithiocarbamato sodium salt, Na(dbzdtc)
Dibenzyldithiocarbamato sodium salt Na(dbzdtc) was prepared based on a literature
[14] procedure with slight modification. Sodium hydroxide (0.05 mol) was dispersed in
10 mL of ethanol and kept at 0 ꢁC. Dibenzylamine (0.05 mol) was added to the ice-cold
mixture at 0 ꢁC and stirred for 30 min before adding 3 mL of cold CS2 (0.5 mol) slowly.
The reaction was left to stir for 3 h while maintaining the temperature. The product
was filtered and dried. Yield: 8.1189 g, (54%), m.p.t (ꢁC) 123.3–124.8. Selected IR,
1
(cmꢀ1): t(C-N) 1443, t(C2-N) 1346, t(C-S)Asym 1144, t(C-S)Sym 969. H NMR (D2O) d/ppm
¼
7.33–7.49 (m, C6H5), 5.42 (s, CH2), 13C NMR (D2O) d ¼ 212.58 (CS2),

JOURNAL OF COORDINATION CHEMISTRY
1247
126.96–128.83 ppm, 136.79 ppm (C), 59.63 ppm (CH2). MS: m/z [Mþ] 272.1446. UV–Vis
ꢂ
ꢂ
p ! p (NCS) 261 nm, p ! p (SCS) 287 nm.
2.2.2. Synthesis of bis(dibenzyldithiocarbamato)Zn(II) complex, [Zn(dbzdtc)2]
[Zn(dbzydtc)2] was prepared using a modified literature method [35]. ZnCl2 (5 mmol)
was dissolved in water and added to 10 mmol aqueous solution of the sodium salt
of dibenzyldithiocarbamate. A white precipitate immediately formed; the reaction
mixture was stirred for 2 h at room temperature. The product was filtered, washed
with water–methanol (1:2 ratio) and dried at room temperature. Yield: 2.3173 g
(78.5%), m.p.t (ꢁC) 186–193, Anal. Calcd. for C30H28N2S4Zn (%): C, 59.05; H, 4.63; N,
4.59; S, 21.02. Found: C, 58.67; H, 4.79; N, 4.54; S, 21.35. Selected IR, (cm1): t(C-N)
1
1482, t(C-S) 941. H NMR (DMSO-d6) d/ppm ¼ 5.08 ppm (s, CH2), 7.33–7.39 ppm (m,
C6H5) and 13C NMR (DMSO-d6) d/ppm ¼ 56.07 ppm (CH2), 128.39–127.24 ppm (CH),
135.29 ppm (CS2), 206.82 ppm. Mass spec Mþ 414.0539 m/z UV–Vis MLCT-285 nm.
Recrystallization of the complex in DCM/hexane gave clear crystals suitable for
X-ray analysis.
2.3. Crystal structure determination and structure refinement
Single yellow block-shaped crystals of [Zn(dbzdtc)2] were recrystallized from DCM/hex-
ane solution by slow evaporation. A Bruker SMART APEX2 area detector diffractometer
was used to collect the crystal data at T ¼ 100 K. ShelXS-2013 [36] was used for space
group (P21/c (# 14)) identification. The structure was solved and refined by least
squares minimization with ShelXL-2016/6 [37]. All non-hydrogen atoms were refined
anisotropically. Hydrogen positions were calculated geometrically and refined using
the riding model.
2.4. In vitro anticancer activity
Cytotoxicity assay of [Zn(dbzdtc)2] in comparison with cisplatin was analyzed using
MTT (3-(4,5-dimethylthiozol-2yl)-2,5-diphenyltetrazolium bromide) assay adapted from
the reported protocol [38]. MCF-7 and HEK293 cells were seeded at a density of
2 ꢃ 103 cells per well grown in 96-well microtiter plates in growth medium (100 mL)
and incubated for 24 h at 37 ꢁC. After which the growth medium was substituted with
a fresh medium of the complex prepared in DMSO at different concentrations (50, 30,
20 and 10 lg/mLꢀ1). The cells were then treated with the complex for 48 h at 37 ꢁC in
triplicate, followed by addition of the incubation growth medium (100 mL) containing
10 mL MTT salt solution (5 mg/mL in PBS) and cells were incubated for 4 h at 37 ꢁC.
DMSO (100 mL) was added after the removal of medium-MTT solution. Cell survival
was measured by absorbance of each well at 540 nm. The experiment controls were
also screened against human embryonic kidney cancer cell HEK293 and human breast
cancer cell MCF-7.

1248
P. A. AJIBADE ET AL.
Figure 1. Molecular structure of bis(dibenzyldithiocarbamato)Zn(II) complex.
3. Results and discussion
3.1. Synthesis
In this study, the ligand was synthesized by reaction of dibenzylamine with CS2 and
sodium hydroxide at low temperature. The white solid obtained was stable and sol-
uble in water, ethanol and methanol. Reaction of two molar equivalents of dibenzyl
dithiocarbamate ligand with one equivalent of zinc(II) chloride gave the Zn(II) dithio-
carbamate complex in high yield. The complex was soluble in dichloromethane and
DMSO. Mass spectrometry of the ligand gave Mþ of 272.1446 m/z and 414.0539 m/z
for the complex, indicating a metal to ligand ratio of 1:2.
3.2. Molecular structure of [Zn(dbzdtc)2]
The molecular structure of [Zn(dbzdtc)2] is shown in Figure 1. Crystal data and struc-
ture refinement are presented in Table 1, while selected bond lengths and angles are
shown in Table 2. [Zn(dbzdtc)2] adopted a four-coordinate mononuclear structure in
which the Zn(II) ion is coordinated to two symmetrical dibenzyl dithiocarbamate
anions. The compound crystallized in the least common monomeric structure of Zn(II)
dithiocarbamate [22, 32–34] due to the steric hinderance imposed on the molecule by
the bulky dibenzyl groups.
Most reports on the crystal structures of Zn(II) dithiocarbamate complexes indicate
they are dimeric or with zinc in a five-coordinate geometry [25, 39, 40]. Decken et al.
[41] reported the crystal structure of bis(N,N-dibenzyldithiocarbamato)zinc(II),
[Zn(bzydtc)2] as a Pbcn orthorhombic system but the current compound crystallizes in
a monoclinic system with a space group P21/C. The bond lengths of Zn-S1 (2.3629(3) Å)
and Zn-S2 (2.3312(3) Å) [41] were slightly different from this current compound with
Zn-S1 (2.3459(4) Å) and Zn-S2 (2.3355(4) Å). The Zn—S bond lengths [2.3573(4),

JOURNAL OF COORDINATION CHEMISTRY
1249
Table 1. Crystal data and structure refinements of bis(dibenzyldi-
thiocarbamato) Zn(II).
Compound
[Zn(dbzdtc)2]
Formula
Dcalc./g cmꢀ3
l/mmꢀ1
Formula weight
Color
C30H28N2S4Zn
1.433
1.187
610.15
Yellow
Block
Shape
Size/mm3
T/K
0.34 ꢃ 0.18 ꢃ 0.12
100
Crystal system
Space group
a/Å
Monoclinic
P21/c
13.1994(7)
9.2486(5)
23.1722(13)
90
91.830(2)
90
2827.3(3)
4
b/Å
c/Å
a/ꢁ
b/ꢁ
c/ꢁ
V/Å3
Z
Z’
1
Wavelength/Å
Radiation type
Hmin/ꢁ
0.71073
MoKa
1.759
27.482
45135
6442
6089
0.0323
334
Hmax/ꢁ
Measured refl.
Independent refl.
Reflections with I > 2(I)
Rint
Parameters
Restraints
Largest peak
Deepest hole
GooF
0
0.399
ꢀ0.386
1.045
wR2 (all data)
wR2
R1 (all data)
R1
0.0633
0.0626
0.0261
0.0245
Table 2. Selected bond lengths (Å) and angles (ꢁ) for bis(dibenzyldithiocarbamato)Zn(II).
Bond length
Bond angle
Atom
Length/Å
Atom
Angle/ꢁ
Zn1-S1
Zn1-S2
Zn1-S3
Zn1-S4
S1-C8
2.3459(4)
2.3355(4)
2.3573(4)
2.3403(4)
1.7323(14)
1.7252(13)
1.7271(13)
1.7320(13)
1.3260(18)
1.3323(17)
S1-Zn1-S2
S3-Zn1-S4
S1-Zn1-S3
S2-Zn1-S4
S1-Zn1-S4
78.307(13)
78.023(13)
119.466(15)
130.800(14)
128.978(14)
128.342(14)
117.50(8)
117.49(8)
81.65(5)
82.10(5)
S2-C8
S2-Zn1-S3
S3-C23
S4-C23
N1-C8
N2-C23
S1—C8—S2
S4—C23—S3
C8—S1—Zn1
C8—S2—Zn1
2.3403(4), 2.3355(4) and 2.3459(4) Å] are almost equal and similar to reported Zn(II)
dithiocarbamate monomeric complexes [22, 32–34].
The most significant difference between the reported compound by Decken et al.
[41] and the current compound is the dithiocarbamato S—Zn—S bite angles. In the

1250
P. A. AJIBADE ET AL.
Figure 2. Packing diagram of bis(dibenzyldithiocarbamato)Zn(II).
current compound, the S1—Zn1—S2 and S3—Zn1—S4 bite angles are 78.307(13)ꢁ
and 78.023(13)ꢁ, respectively, which leads to enlargement of other perinuclear angles
to 119.466(15)ꢁ–130.800(14)ꢁ, consequently, deviating from the ideal 109.5ꢁ for tetra-
hedral geometry [25]. Whereas in the reported bis(N,N-dibenzyldithiocarbamato)zinc(II)
described as distorted tetrahedral geometry, the S1–Zn–S2 (78.043(11)) and S1–Zn–S2i
(123.975(12)) [41] bond angles are not symmetrical with one small bite angle and
while the other was greater than 120ꢁ. In order to determine whether the molecular
structure of the present [Zn(dbzdtc)2] is tetrahedral, see-saw, square planar or trigonal
pyramidal, the geometric parameter for four-coordinate compounds, s4 proposed by
Houser et al. [42] which quantitatively evaluates the geometry of four-coordinate com-
plexes was used. The geometric parameter is given by s4 ¼ 360ꢀðaþbÞ , where a and b
141
are the two largest h angles. The calculated s4 value of 0.72 for the [Zn(dbzdtc)2] indi-
cates the molecular structure could best be described as see-saw geometry. The C—S
bond ranges from 1.7252(13) to 1.7323(14) Å with a mean value of 1.7291 Å, shorter
than the 1.8 Å observed in a typical C—S single bond and longer than a typical C—S
double bond. This suggests that all the C—S bonds in the complex have partial dou-
ble bond character common in dithiocarbamate compounds.
The bond lengths for N1—C8 and N2—C23 are 1.3260(18) and 1.3323(17) Å,
respectively, which are between a typical single N—C (1.47 Å) and N—C double bond
(1.28 Å) because of its partial double bond attributes, suggest the delocalization of
p–electrons in the thioureide moiety [43]. The packing diagram of the complex in a
unit cell viewed down the b-axis (Figure 2) shows four monomeric complexes within
the crystal packing. The molecular packing is stabilized by C-H–-S and C-H–-p contacts
forming a head-to-tail zigzag layer. The C-H–-S contacts were 3.388, 3.496 and 2.862 Å

JOURNAL OF COORDINATION CHEMISTRY
1251
[Zn(dbzdtc)2]
14
12
10
8
6
4
2
0
50μg/μl
30μg/μl
20μg/μl
10μg/μl
CONCENTRATION
HEK293 MCF7
Figure 3. Cytotoxic activity of bis(dibenzyldithiocarbamato)Zn(II) against MCF7 and HEK293
cell lines.
while C-H–-p contacts were 2.786 Å. These contacts are slightly greater than those pre-
viously reported [20, 44].
3.3. Spectra studies of the dibenzyldithiocarbamate ligand and
bis(dibenzyldithiocarbamato) Zn(II) complex
FTIR spectra of the ligand and complex were compared and assigned. In the FTIR
spectrum of the ligand (Figure S1), the peaks at 1443 and 1346 cmꢀ1 are assigned to
the thiouride ꢀ(C—N) and ꢀ(C2—N) stretching vibrations, respectively. These vibrations
indicate an electron flow from nitrogen to sulfur through a planar delocalized p-orbital
system [45]. In the complex (Figure S2), there is only one band observed for ꢀ(C—N)
vibration at 1482 cmꢀ1 which is shifted to higher frequency compared to the ligand
and further confirms the delocalization of the thioureide moiety toward the zinc cen-
ter. Two bands assigned to ꢀ(C—S)Sym and ꢀ(C—S)Asym were observed at 969 and
1144 cmꢀ1 in the spectrum of the ligand but in the complex it appeared as a single
band at 941 cmꢀ1, which indicates that the ligand coordinates to the metal center
through the sulfurs as a bidentate ligand [46].
1
The H NMR spectrum of the ligand (Figure S3) shows two peaks, one singlet at
5.42 ppm assigned to the methylene protons and a multiplet at 7.33–7.49 ppm
assigned to the benzene ring protons. These peaks shifted to 5.08 ppm for the methy-
lene protons and 7.35–7.41 ppm for benzene ring protons for the complex (Figure S4).
The observed shifts are due to the coordination to Zn(II). The 13C NMR spectrum of
the ligand (Figure S5) shows a peak at 56.32 ppm which could be assigned to methy-
lene group carbon bonded to the nitrogen; this peak is observed in Figure S6 at
56.82 ppm in the complex. The peaks assigned to aryl methine carbons are observed
between 128.83–126.96 for the ligand and at 127.24–128.39 for the complex. The peak
for a quaternary carbon on a benzene ring is observed at 136.65 ppm on the ligand

1252
P. A. AJIBADE ET AL.
Table 3. IC50 (lM) values of the complex and cisplatin against MCF7
and HEK293 cell lines.
IC50 (lM)
Compounds
MCF7
HEK293
[Zn(dbzdtc)2]
Cisplatin
6.408
2.5
4.423
3.2
and at 136.03 ppm on the complex. These observed shifts are an indication of com-
plexation. Decken et al. [41] synthesized bis(N,N-dibenzyldithiocarbamate) zinc(II) and
found CH2 hydrogen at 5.08 ppm and carbon at 55.7 ppm, which are not too different
from the present compound.
3.4. Anticancer activity studies
The cytotoxic activity of [Zn(dbzdtc)2] was tested against MCF7 and HEK239 cell lines
using MTT assay at different concentrations. Figure 3 shows the cytotoxicity to be con-
centration-dependent with a concentration of 10 lg/lL smallest activity against MCF7
and HEK293 cells. At other concentrations, [Zn(bzydtc)2] showed higher cytotoxicity
against MCF7 cells, exhibiting considerable selectivity. Cell growth inhibition of 50%
(IC50) was determined by non-regression analysis using GraphPad Prism software.
Table 3 presents the IC50 values. The IC50 value of the complex against MCF7 cells was
6.41 lM, showing potent cytotoxicity although it was less active in vitro in comparison
to cisplatin. The activity of the complex against HEK293 cell line which are non-cancer
cells is 4.423 lM; this is slightly less than cisplatin. This lower activity may imply that
this compound may not cause severe side effects on normal cells.
4. Conclusion
Bis(dibenzyldithiocarbamato)Zn(II) was synthesized and characterized by spectroscopic
techniques and single crystal X-ray crystallography. The molecular structure of the
compound revealed Zn(II) bonded to two dibenzyl dithiocarbamate anions to form a
four-coordinate see-saw geometry. Cytotoxic activity of the compound was tested
against breast adenocarcinoma (MCF-7) cancer cell line and embryonic kidney
(HEK293) cells at four different concentrations. The cytotoxicity studies reveal that the
complex was less potent than cisplatin, but active against MCF-7 breast cancer cells
with IC50 value of 6.41 and 4.423 lM against HEK293 cell line.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
The authors acknowledge the award of research grant by National Research Foundation,
South Africa.

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1253
ORCID
Peter A. Ajibade
http://orcid.org/0000-0002-8581-2387
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