Synthesis and structural studies of nickel(II)- and copper(II)-N,N′-diarylformamidine dithiocarbamate complexes as antimicrobial and antioxidant agents

Article abstract of DOI:10.1016/j.poly.2019.06.038

A series of six N′N-diarylformamidine dithiocarbamate ligands and their metal complexes of Cu(II) and Ni(II) chloride salts have been synthesized. Three symmetrical, N,N′-bis(2,6-dimethylphenyl)formamidine dithiocarbamate (DL1), N,N′-bis(2,6-disopropylphe

Full text of DOI:10.1016/j.poly.2019.06.038

Accepted Manuscript
Synthesis and structural studies of nickel(II)- and copper(II)-N,N'-diarylforma-
midine dithiocarbamate complexes as antimicrobial and antioxidant agents
Segun D. Oladipo, Bernard Omondi, Chunderika Mocktar
PII:
S0277-5387(19)30449-8
https://doi.org/10.1016/j.poly.2019.06.038
POLY 14029
DOI:
Reference:
Polyhedron
To appear in:
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Accepted Date:
22 May 2019
14 June 2019
Please cite this article as: S.D. Oladipo, B. Omondi, C. Mocktar, Synthesis and structural studies of nickel(II)- and
copper(II)-N,N'-diarylformamidine dithiocarbamate complexes as antimicrobial and antioxidant agents,
Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.2019.06.038
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Synthesis and structural studies of nickel(II)- and copper(II)-N,N'-diarylformamidine
dithiocarbamate complexes as antimicrobial and antioxidant agents
Segun D. Oladipo,a Bernard Omondi*b and Chunderika Mocktar c
a School of Chemistry and Physics, Westville Campus, University of Kwazulu-Natal, Private
Bag X54001, Durban, 4000, South Africa.
b School of Chemistry and Physics, Pietermaritzburg Campus, University of Kwazulu-Natal,
Private Bag X01, Scottsville, 3209, South Africa.
c Discipline of Pharmaceutical Sciences, School of Health Sciences, University of Kwazulu-
Natal, Private Bag X54001, Durban, 4000, South Africa.
*Corresponding author
E-mail: owaga@ukzn.ac.za Tel: (+27) 332606436
ABSTRACT
A series of six N'N-diarylformamidine dithiocarbamate ligands and their metal complexes of
Cu(II) and Ni(II) chloride salts have been synthesized. Three symmetrical, N,N'-bis(2,6-
dimethylphenyl)formamidine dithiocarbamate (DL1), N,N'-bis(2,6-disopropylphenyl)
formamidine dithiocarbamate (DL2), N,N'-mesityl formamidine dithiocarbamate (DL3) and
three
unsymmetrical
N'-(2,6-dichlorophenyl-N-(2,6-dimethylphenyl)
formamidine
dithiocarbamate (DL4), N'-(2,6-dichlorophenyl)-N-(2,6-diisopropylphenyl) formamidine
dithiocarbamate (DL5) and N'-(2,6-dichlorophenyl)-N-mesityl formamidine dithiocarbamate
(DL6) dithiocarbamate ligands were reacted with CuCl2 and NiCl2 to give [Ni-(DL1)2] (1),
[Ni-(DL2)2].H2O (2), [Ni-(DL3)2] (3), [Ni-(DL4)2].3H2O (4), [Ni-(DL5)2].H2O (5), [Ni-
(DL6)2] (6), [Cu-(DL1)2].3H2O (7), [Cu-(DL2)2] (8), [Cu-(DL3)2].2H2O (9), [Cu-
(DL4)2].3H2O (10), [Cu-(DL5)2].3H2O (11) and [Cu-(DL6)2] (12). All ligands and the
complexes were characterized using FT-IR, UV-vis, 1H and 13C NMR, mass spectrometry and
by elemental analysis. In addition, the structures of complexes 1, 5, 8 and 11 were confirmed
by single crystal X-ray diffraction analysis as mononuclear neutral species in which the
geometry around the metal centers is distorted square planar. In this coordination manner, the
metal centers bound to four sulfur atoms from two dithiocarbamate ligand in a bidentate
fashion. All complexes showed moderate to good antibacterial activities against Gram-
negative, Salmonella typhimurium, Pseudomonas aeruginosa, Escherichia coli and Klebsiella
1

pneumoniae and gram-positive, Staphylococcus aureus (methicillin resistant) and
Staphylococcus aureus bacteria. Complexes 4 and 5 were found to be more active than
ciprofloxacin against E. coli and K. pneumoniae. In addition, complexes with chloro substituted
ligands displayed higher activities. Antioxidant activities of complexes with symmetrical
formamidine ligands were generally high compared to the ones with the unsymmetrical
formamidine moeities, with 2 having an IC50 value of 1.10 x 10-3 mM and 12 had 2.91 mM.
Keywords: N,N'-diarylformamidine; Dithiocarbamates; Metal complexes; Antioxidant;
Antibacterial
1.0
INTRODUCTION
The increase in the number of multi-drug resistant microbial pathogens and the emergence of
new ones has made treatment of infectious diseases a challenge in the last decade [1].
Development of new drugs endowed with excellent antimicrobial activities, and whose
mechanism of action is distinct from those of well-known antimicrobial agents to which
relevant pathogens are now resistant to is very important [2]. Due to the presence of multiple
intrinsic and acquired mechanism of antimicrobial resistance, controlling the spread of deadly
microbes in various healthcare settings is getting difficult [3] and for this reason, research on
new drugs that can kill drug resistant bacteria is on the increase. Of importance is also research
on antioxidant activity of synthetic and natural compounds. This is because of the health
benefits that antioxidants have in protecting organisms and cells from damage induced by
oxidative stress [4]. Oxidative stress is caused by the presence of reactive oxygen species
•-
(ROS) such as superoxide radical anion (O2 ), hydroxyl radical (OH•), and hydrogen peroxide
(H2O2) often formed by the partial reduction of dioxygen (O2) species [5]. Of the damage
caused by ROS in the body is normally to proteins, lipids and DNA thereby accelerating ageing,
causing cancerous inflammations and, also cardiovascular and neurodegenerative diseases
[5].Various methods of analysis have been adopted to study antioxidant activity. For example,
for in vitro antioxidant test, the scavenging activity of DPPH (2,2-diphenyl-1-picrylhydrazyl)
assay as a stable free radical is often used for both natural and synthetic compounds [6].
Dithiocarbamates and their metal complexes have been tested as anticancer [7, 8],
antimicrobial [9, 10], and as antioxidant agents [11, 12]. Dithiocarbamates are often
synthesized by the reaction of either primary or secondary amines with carbon disulfide in the
presence of a base, while their metal complexes are prepared via simple ligand displacement
reactions following the addition of the dithiocarbamate salt to a metal precursor in the
2

appropriate ratio [13-15]. The electronic properties, structural architectures and applications
are influenced by the easily electronically tunable amines used in their synthesis [16].
Biological studies of Ni(II) and Cu(II) dithiocarbamate complexes have been reported but not
in much detail [8, 17-19]. Mamba et al. [9] reported the in vitro antimicrobial studies of
cyclohexylamine-N-dithiocarbamate transition metal complexes and their research showed that
the antimicrobial activity of the free ligand was enhanced upon chelation with metal ions. Their
study showed that Ni(II) complexes displayed antimicrobial activity against Pseudomonas
aeruginosa and Staphylococcus aureus. Antibacterial activities of 2-amino pyridine
dithiocarbamate ligand and its Cu(II) and Co(II) complexes was also reported by Gopal et al.
[20]. Their study showed that the complexes moderately inhibited the growth of Escherichia
coli, S aureus and Bacillus subtilis and the Cu(II) complex showed better activity as compared
to the Co(II) complex. Recently, Onwudiwe et al. [11] reported the antioxidant activity of
sodium N-ethyl-N-phenyldithiocarbamate and its Cu(II) complexes. Their results showed that
the complex had scavenging activity of 75 % at 500 µg/ml relatively to the ligand which has
42 % at the same concentration. Herein, we report the synthesis, characterization, antioxidant
and antimicrobial studies of symmetrical and unsymmetrical Ni(II) and Cu(II) N,N'-
diarylformamidine dithiocarbamate complexes. All compounds were characterized by UV-
Visible, FT-IR, Mass and NMR spectrometry and single crystal X-ray structures of two of the
Cu(II) and two of the Ni(II) complexes. The antimicrobial potentials of the metal complexes
were evaluated against Gram-negative bacteria strains {E coli, S. typhimurium P. aeruginosa
and K pneumoniae and Gram-positive bacteria strains {MRSA and S. aureus}.
2.0
2.1
Experimental section
Materials
All solvents (ACS reagent grades ≥ 99.5 %) were obtained from Sigma-Aldrich and used as
obtained without further purification. Reagents: 2,6-diisopropylaniline (97%), 2,6-
dimethylaniline (99 %), 2,4,6-trimethylaniline (98 %), 2,6-dichloroaniline (98 %), triethyl
orthoformate (99 %) and carbon disulfide were also obtained from Sigma Aldrich. Metal salts:
CuCl2. 2H2O (97 %), NiCl2. 6H2O (98 %) and KOH (85 %) were obtained from Promark
Chemicals South Africa.
3

2.2
Instrumentation
1H and 13C NMR spectra were recorded at 25 °C on a Bruker AvanceIII 400 MHz spectrometer.
1
13
Both H NMR and C NMR data were recorded in either CDCl3 referenced to the residual
CDCl3 peaks at δ 7.26 and δ 77.00 ppm or (CD3)2SO (DMSO) referenced to the residual
(CD3)2SO peaks at δ 2.50 and δ 39.52 ppm respectively. Elemental analyses were recorded on
a Vario elemental EL cube CHNS analyzer. IR spectra were obtained on a PerkinElmer
Universal ATR spectrum 100 FT-IR spectrometer. Mass spectra of complexes were obtained
from a Water synaptic GR electrospray positive spectrometer and UV-Vis absorption spectra
was recorded on Shimadzu UV-vis-NIR spectrophotometer.
3.0
3.1
General methods of synthesis
Synthesis of symmetrical formamidine ligand (secondary amine)
N,N'-bis(2,6-dimethyphenyl)formamidine (L1), N,N'-bis(2,6-diisopropylphenyl)formamidine
(L2) and N,N`-bis(2,4,6-trimethylphenyl)formamidine were synthesized following literature
procedure with a slight modification [21]. Three or four drops of acetic acid were added to the
reaction mixture in a round-bottom flask charged with the aniline (2 mole equivalents) and
triethyl orthoformate (1 mole equivalent). The reaction mixture was heated under reflux and
temperature maintained at 130-150 °C. After 3hr, the temperature was increased to 160 °C to
remove all volatiles via distillation. The reaction was allowed to cool and the crude product
was triturated with hexane and collected by gravity filtration. The solids obtained were
recrystallized in 50 cm3 of hot acetone and stored at 4 °C to yield the pure products.
3.2
Synthesis of unsymmetrical formamidine ligand (secondary amine)
Unsymmetrical formamidine were synthesized as follows: three or four drops of acetic acid
were added to the reaction mixture containing aniline (30 mmol) and triethyl orthoformate (30
mmol) in a round-bottom flask. The mixture was refluxed at 140 °C for 30 min with stirring
followed by distillation (2 mmol of EtOH collected). The second aniline (30 mmol) was then
added to the reaction mixture and heating continued until 1 mmol of ethanol was collected.
Upon cooling to 25 °C, the solution solidified. The crude product was triturated with cold
hexane and collected by filtration. Solids were then washed with 50 cm3 of acetone to
completely remove traces of symmetrical formamidine by-products that were formed during
4

the reaction to afford N-(2,6-dichlorophenyl)-N-(2,6-dimethylphenyl)formamidine, (L4), N-
(2,6-dichlorophenyl)-N-(2,6-dimethylphenyl)formamidine (L5) and N-(2,6-dichlorophenyl)-
N-mesitylformamidine (L6).
3.3
Synthesis of N,N'-diarylformamidine dithiocarbamates
An equimolar ratio of the formamidines and KOH in 20 ml of acetonitrile was stirred for 20
min in a cold mixture of ice at 0–5 °C. to form a white color solution. To the resulting solution,
carbon disulfide of the same molar ratio was added drop-wise and stirring continued for 3hr in
an ice bath and another one hour at room temperature to give a yellow solution. The solvent
was removed using rotary evaporator and a yellow crude solid product was obtained. The crude
product was rinsed with ethanol three times to remove unreacted formamidine to give a pure
yellow solid product and stored in a desiccator.
3.3.1 Synthesis of DL1
The reaction of L1 (1.00 g, 4 mmol), KOH (0.22 g, 4 mmol) and CS2 (0.24 ml, 4 mmol) in 20
ml of acetonitrile furnished dithiocarbamate ligand DL1 as a yellow powder. Yield 87 %.
Decomposition tempt. range, 237 – 242 °C. 1H NMR (DMSO, 400 MHz): δ (ppm) 1.90 (s, 6H,
CH3-Ar), 1.99 (s, 6H, CH3-Ar), 6.91 (s, 3H, Ar-H), 6.85 (d, 2H, J HH = 7.48 Hz, Ar-H), 6.70 (t,
1H, JHH = 7.48 Hz, Ar-H) 9.86 (s, 1H, -CH=N) . 13C NMR (DMSO, 100 MHz) δ (ppm) 16.7,
17.3, 116.9, 121.0, 124.9, 126.0, 126.4, 126.5, 126.6, 134.1, 140.9, 148.3, 151.5, 217.62. IR υ
(cm-1) 2918(w), 1640(s), 1467(s), 1000(s), 921(w). UV-Vis (CHCl3, λmax, nm), 292, 340. Anal.
calcd. for C18H22KN2OS2: C, 56.21; H, 5.50; N, 7.28; S, 16.67. Found: C, 56.06; H, 5.75; N,
7.22; S, 16.20.
3.3.2 Synthesis of DL2
The reaction of L2 (1.46 g, 4 mmol), KOH (0.22 g, 4 mmol) and CS2 (0.16 mL, 4 mmol) in 20
ml of acetonitrile furnished dithiocarbamate ligand DL2 as a yellow powder. Yield 74.18 %.
1
Decomposition tempt. range, 244 – 249 °C. H NMR (DMSO, 400 MHz) : δ (ppm) 1.06 (d,
12H, JHH = 6.88 Hz, -CH3-CH-), 2.84 (m, 2H, JHH = 6.72 Hz, CH-CH3), 2.97 (m, 2H, JHH
=
6.84 Hz, CH-CH3), 6.94 (t, 1H, JHH = 6.76 Hz, Ar-H), 7.02 (d, 2H, JHH = 7.12 Hz, Ar-H), 7.11
13
(d, 2H, JHH = 7.5 Hz, Ar-H), 7.21 (t, 1H, JHH = 7.00 Hz, Ar-H), 10.15 (s, 1H, -CH=N). C
NMR (DMSO, 100 MHz) δ (ppm): 23.80, 24.00, 24.86, 26.58, 28.24, 122.47, 122.86, 127.08,
138.61, 138.87, 145.23, 147.49, 154.21, 220.94. IR υ (cm-1) :2959(s), 2865(w) 1639(s),
5

1452(s), 1152(s), 999(s). UV-Vis (CHCl3, λmax, nm), 293, 339. Anal. calcd. for C26H35KN2S2:
C, 66.08; H, 7.69; N, 5.85; S, 13.39. Found: C, 65.92; H, 7.37; N, 5.85; S, 12.20.
3.3.3 Synthesis of DL3
The reaction of L3 (1.12 g, 4 mmol), KOH (0.22 g, 4 mmol) and CS2 (0.21 mL, 4 mmol) in 20
ml of acetonitrile furnished dithiocarbamate ligand DL3 as a yellow powder. Yield 93.63%.
Decomposition tempt. range, 258 – 263 °C. 1H NMR (DMSO, 400 MHz) δ (ppm) 1.94 (s, 6H,
CH3-Ar), 2.03 (s, 6H, CH3-Ar), 2.15 (s, 3H, CH3-Ar), 2.22 (s, 3H, CH3-Ar) 6.75 (s, 2H, Ar-H),
6.81 (s, 2H, Ar-H), 9.92 (s, 1H, CH=N). 13C NMR (DMSO, 100 MHz) δ (ppm): 13.92, 17.70,
18.32, 20.29, 20.58, 127.26, 127.90, 128.30, 130.51, 134.82, 139.55, 147.10, 152.79, 218.95.
IR υ (cm-1) 2951 (m), 1629(s), 1477(m), 1023(s), 956(w). UV-Vis (CHCl3, λmax, nm), 289,
338. Anal. calcd for C20H23KN2S2: C, 60.87; H, 5.87; N, 7.10; S, 16.25. Found: C, 60.41; H,
5.93; N, 6.95; S, 15.97.
3.3.4 Synthesis of DL4
The reaction of L4 (1.17 g, 4 mmol), KOH (0.22 g, 4 mmol) and CS2 (0.25 g, 4mmol) in 20 ml
of acetonitrile furnished dithiocarbamate ligand DL4 as a yellow powder. Yield 77 %.
1
Decomposition tempt. range, 245 – 248 °C. H NMR (DMSO, 400 MHz): δ (ppm): 2.13 (s,
6H, CH3-Ar), 7.02 (s, 4H, Ar-H), 7.39 (d, 2H, JHH = 8.08 Hz, Ar-H) 10.12 (s, 1H, -CH=N). 13C
NMR (DMSO, 100 MHz) δ (ppm): 17.76, 124.13, 126.37, 126.40, 127.26, 127.52, 127.73,
128.34, 135.23, 135.42, 141.29, 146.08, 154.81, 218.82. IR υ (cm-1): 3177(w), 1614(s),
1432(s), 1128(s), 1033(s). UV-Vis (CHCl3, λmax, nm), 293, 345. Anal. calcd for
C16Cl2H15KN2OS2: C, 45.67; H, 3.95; N, 6.58; S, 15.20. Found: C, 45.52; H, 3.81; N, 6.29; S,
15.19.
3.3.5 Synthesis of DL5
The reaction of L5 (1.40 g, 4 mmol), KOH (0.22 g, 4 mmol) and CS2 (0.21 g, 4 mmol) in 20ml
of acetonitrile furnished dithiocarbamate ligand DL5 as a yellow powder. Yield 91 %.
1
Decomposition tempt. range, 250 – 260 °C. H NMR (DMSO, 400 MHz): δ (ppm): 1.14 (t,
12H, JHH = 7.28 Hz, CH3-CH), 2.87 (m, 2H, JHH = 6.76 Hz, CH-Ar), 6.98 (t, 1H, JHH = 8.08
Hz, Ar-H), 7.10 (d, 2H, JHH = 7.56 Hz, Ar-H), 7.21 (t, 1H, JHH = 7.24 Hz, Ar-H), 7.38 (d, 2H,
JHH = 8.08 Hz, Ar-H) 10.39 (s, 1H, -CH=N-Ar). 13C NMR (DMSO, 100 MHz) δ (ppm): 26.81,
27.75, 27.98, 132.87, 133.98, 137.90, 138.48, 145.31, 146.14, 146.33, 151.41, 156.87, 217.02.
6

IR υ (cm-1): 2960 (w), 1603(s), 1430(s), 1145(s), 1036(s). UV-Vis (CHCl3, λmax, nm), 300,
345. Anal. calcd for C20Cl2H27KN2O3S2: C, 46.61; H, 5.62; N, 5.41; S, 12.63. Found: C, 46.53;
H, 5.55; N, 5.05; S, 12.59.
3.3.6 Synthesis of DL6
The reaction of L6 (1.23 g, 4 mmol), KOH (0.22 g, 4 mmol) and CS2 (0.24 g, 4 mmol) in 20
ml of acetonitrile furnished dithiocarbamate ligand DL6 as a yellow powder. Yield 83 %.
1
Decomposition tempt. range, 255 – 258 °C. H NMR (DMSO, 400 MHz): δ (ppm): 2.07 (s,
6H, CH3-Ar-N-), 2.23 (s, 3H, CH3-Ar), 6.81 (s, 2H, Ar-H) 7.00 (t, 1H, JHH = 8.4 Hz, Ar-H),
7.35 (d, 2H, JHH = 8.04 Hz, Ar-H), 10.11 (s, 1H,CH=N-Ar). 13C NMR (DMSO, 100 MHz) δ
(ppm): 17.68, 20.59, 124.04, 126.34, 127.97, 128.32, 135.02, 135.12, 138.71, 146.17, 154.83,
219.04. IR υ (cm-1) 2915(w), 1612(s), 1435(s), 1141(s), 1032(s). UV-Vis (CHCl3, λmax, nm),
295 343. Anal. calcd for C17Cl2H17KN2OS2: C, 46.46; H, 3.90; N, 6.37; S, 14.59. Found: C,
46.29; H, 4.05; N, 6.33; S, 14.26.
3.4
Synthesis of N,N'-diarylformamidines dithiocarbamate metal complexes
The respective dithiocarbamate salts (2 mmol) were dissolved in 15 mL of acetonitrile. To the
resulting mixture, (1 mmol) of the metal chloride salt dissolved in 10 ml of water was added
drop-wise and stirred for about 30 min at room temperature. A sudden change in color was
observed depending on the nature of the metal used and a precipitate was formed immediately.
The product formed was collected by filtration. The Complexes were then recrystallized in hot
ethanol at 80 °C to remove any unreacted ligand. The solid precipitate was then collected by
filtration and dried in the oven at 50 °C.
3.4.1 Synthesis of [Ni-(DL1)2] (1)
The reaction of DL1 (0.30 g, 0.8 mmol) and NiCl2.6H2O (0.10 g 0.4 mmol) in acetonitrile
furnished complex 1 as a purple-red powder. Yield 72 %. Decomposition temperature range,
265-267 °C. 1H NMR (CDCl3, 400 MHz) δ (ppm): 2.09 (s, 12H, CH3-Ar), 2.29 (s, 12H, CH3-
Ar), 6.91 (t, 2H, JHH = 7.24 Hz, Ar-H), 7.00 (d, 5H, JHH = 7.32 Hz, Ar-H), 7.19 (d, 5H, JHH
=
13
7.28 Hz, Ar-H), 8.85 (s, 2H, -CH=N) : C NMR (CDCl3, 100 MHz) δ (ppm): 17.62, 18.64,
18.71, 124.32, 126.33, 127.61, 128.24, 128.85, 129.91, 133.96, 135.91, 145.04, 146.66, 216.95.
IR υ (cm-1): 2949(m), 1647(s), 1474(m), 1126(m), 999(m), 376(s). ESI-TOF MS: m/z (%);
7

[M]+ 713.14. UV-Vis (CHCl3, λmax, nm), 262, 341, 457. Anal. calcd. for C36H40N4NiS4: C,
60.42; H, 5.63; N, 7.83; S, 17.92. Found: C, 60.28; H, 5.29; N, 7.71; S, 17.35
3.4.2 Synthesis of [Ni-(DL2)2].H2O (2)
The reaction of DL2 (0.3 g, 0.6 mmol) and NiCl2.6H2O (0.07 g, 0.3 mmol) in acetonitrile
furnished complex 2 as an orange powder. Yield 68.25 %. Decomposition temp. range, 280 –
282 °C. 1H (CDCl3, 400 MHz) : δ (ppm) 1.14 (d, 26H, JHH = 5.2 Hz, -CH3-CH), 1.32 (s, 10H,
CH3-), 1.40 (s, 12H, -CH3), 2.83 (m, 8H CH-CH3) 7.10 (s, 5H, Ar-H), 7.15 (s, 7H, Ar-H), 8.88
13
(s, 2H, -CH=N). C NMR (CDCl3, 100 MHz) δ (ppm): 24.05, 24.10, 25.26, 27.61, 29.26,
30.93, 123.19, 124.61, 124.85, 130.57, 138.74, 145.14, 146.26, 206.98 IR υ (cm-1) 2961(s),
2867(w) 1651(s), 1460(s), 1178(s), 996(s), 378(s). ESI-TOF MS: m/z (%); [M + K]+. 973.30.
UV-Vis (CHCl3, λmax, nm), 261, 341, 457. Anal. calcd. for C52H74N4NiOS4: C, 65.73; H, 7.80;
N, 5.85; S, 13.38. Found: C, 65.60; H, 7.80; N, 5.70; S, 13.06.
3.4.3 Synthesis of [Ni-(DL3)2] (3)
The reaction of DL3 (0.3 g, 0. 8mmol) and NiCl2.6H2O (0.08 g, 0.4 mmol) in acetonitrile
furnished complex 3 as a purple-red powder. Yield 70 %. Decomposition temp. range, 270-
272 °C. 1H NMR (CDCl3, 400 MHz): δ (ppm) 2.05 (s, 10H, CH3-Ar), 2.24 (s, 18H, CH3-Ar),
2.30 (s, 3H, CH3-Ar), 6.82 (s, 4H, Ar-H), 7.00 (s, 4H, Ar-H), 8.81 (s, 2H, CH=N). 13C NMR
(CDCl3, 150 MHz) δ (ppm): 17.58, 17.57, 18.52, 19.26, 20.63, 21.21, 127.47, 128.73, 128.87,
129.06, 129.50, 129.61, 133.61, 135.43, 144.79, 217.92. IR υ (cm-1) 2914(w), 1644(s), 1477(m)
1145(m), 955(w), 318(s). ESI-TOF MS: m/z (%); [DL3 – CH3]+ 340.18. UV-Vis (CHCl3,
λmax, nm), 258, 340, 454. Anal. calcd. for C40H48N4NiS4: C, 62.25; H, 6.27; N, 7.26; S, 16.62.
Found: C, 61.90; H, 6.11; N, 7.09; S, 16.60.
3.4.4 Synthesis of [Ni-(DL4)2].3H2O (4)
The reaction of DL4 (0.20 g, 0.50 mmol) and NiCl2.6H2O (0.06 g 0.25 mmol) in acetonitrile
furnished complex 4 as a purple-red powder. Yield 78 %. Decomposition temp. range 260-262
°C. 1H NMR (CDCl3, 400 MHz) δ (ppm): 2.31 (s, 12H, CH3-Ar-), 6.96 (t, 3H, JHH = 8.16 Hz,
Ar-H), 7.16 (d, 4H, JHH = 7.4 Hz, Ar-H), 7.28 (s, 3H, Ar-H), 8.94 (s, 2H, -CH=N). : 13C NMR
(CDCl3, 100 MHz) δ (ppm): 17.59, 125.42, 126.57, 128.32, 128.78, 130.09, 133.36, 136.20,
143.61, 147.33, 219.32. IR υ (cm-1) 2968(w), 1642(s), 1435(m), 1096(m), 887(s).371(s). ESI-
TOF MS: m/z (%); [M – 2Cl]+. 723.03. UV-Vis (CHCl3, λmax, nm), 263, 341, 462. Anal. calcd.
8

for C32Cl4H34N4NiO3S4: C, 45.52; H, 4.03; N, 6.58; S, 15.19. Found: C, 45.47; H, 3.95; N,
6.42; S, 15.18.
3.3.5 Synthesis of [Ni-(DL5)2].H2O (5)
The reaction of DL5 (0.20 g, 0.4 mmol) and NiCl2.6H2O (0.05 g 0.20 mmol) in acetonitrile
furnished complex 5 as an orange powder. Yield 78 %. Decomposition tempt. Range, 274-276
°C. 1H NMR (CDCl3, 400 MHz) δ (ppm): 1.25 (s, 16H, CH3-CH), 1.34 (s, 8H, CH3-CH), 2.85
(s, 4H, CH-CH3, 6.93 (s, 2H, Ar-H), 7.55 (s, 2H, Ar-H), 9.10 (s, 2H, -CH=N). 13C NMR
(CDCl3, 100 MHz) δ (ppm): 17.51, 29.26, 30.93, 123.16, 124.61, 124.85, 130.56, 138.75,
146.26, 218.76. IR υ (cm-1) 2961(w), 1639(s), 1435(m), 1084(m), 888(s).372(s). ESI-TOF MS:
m/z (%); [M – 2Cl]+. 835.15. UV-Vis (CHCl3, λmax, nm), 262, 343, 467. Anal. calcd. for
C40Cl4H46N4NiOS4: C, 51.80; H, 5.00; N, 6.04; S, 13.38. Found: C, 51.21; H, 4.63; N, 5.84; S,
13.43.
3.3.6 Synthesis of [Ni-(DL6)2] (6)
The reaction of DL6 (0.20 g, 0.50 mmol) and NiCl2.6H2O (0.06 g 0.25 mmol) in acetonitrile
furnished complex 6 as a purple-red powder. Yield 80 %. Decomposition tempt. Range, 268-
1
270 °C. H NMR (CDCl3, 400 MHz) δ (ppm): 2.26 (s, 12H, CH3-Ar), 2.29 (s, 6H, CH3-Ar),
6.93 (d, 2H, JHH = 7.7, Ar-H), 6.98 (s 4H, Ar-H), 8.91 (s, 2H, -CH=N). 13C NMR (CDCl3, 100
MHz) δ (ppm): 17.51, 29.26, 30.93, 123.19, 124.61, 124.85, 130.56, 138.75, 146.26, 218.76.
IR υ (cm-1) 2970(w), 1644(s), 1436(m), 1098(m), 889(s).317(s). ESI-TOF MS: m/z (%); [M –
2Cl]+. 752.38. UV-Vis (CHCl3, λmax, nm), 263, 343, 465. Anal. calcd. for C34H32Cl4N4NiS4:
C, 49.48; H, 3.91; N, 6.79; S, 15.54. Found: C, 49.02; H, 3.47; N, 6.61; S, 15.27.
3.3.7 Synthesis of [Cu-(DL1)2].3H2O (7)
The reaction of DL1 (0.3 g, 0.8 mmol) and CuCl2.2H2O (0.07 g, 0.4 mmol) in acetonitrile
furnished complex 7 as a deep brown powder. Yield 89 %. Decomposition tempt. Range, 280–
282 °C. IR υ (cm-1) 2918(w), 1645(s), 1470(m), 1186(s), 999(m), 374(w). ESI-TOF MS: m/z
(%); [M]+ 718.13. UV-Vis (CHCl3, λmax, nm), 300, 316 (shoulder), 449. Anal. calcd. for
C36CuH46N4O3S4: C, 56.21; H, 5.99; N, 7.33; S, 17.26. Found: C, 56.04; H, 5.99; N, 7.27; S,
17.15.
9

3.3.8 Synthesis of [Cu-(DL2)2] (8)
The reaction of DL2 (0.3 g, 0.6 mmol) and CuCl2.2H2O (0.50 g, 0.3 mmol) in acetonitrile
furnished complex 8 as a reddish-brown powder. Yield 84 %. Decomposition tempt. Range,
294–296 °C. IR υ (cm-1) 2960(m), 2915(m), 1639(s), 1477(m), 1143(m) 957(w), 395(w). ESI-
TOF MS: m/z (%); [M + K – (DL2 – 5H)]+ 537.09. UV-Vis (CHCl3, λmax, nm), 301, 319
(shoulder), 449. Anal. calcd. for C52CuH72N4S4: C, 60.69; H, 5.98; N, 8.18; S, 18.50. Found:
C, 60.38; H, 5.97; N, 8.12; S, 18.11.
3.3.9 Synthesis of [Cu-DL3)2].2H2O (9)
The reaction of DL3 (0.3 g, 7.3mmol) and CuCl2.2H2O (0.06 g, 3.6mmol) in acetonitrile
furnished complex 9 as a deep brown powder. Yield 86 %. Decomposition tempt. range, 290–
293 °C. IR υ (cm-1) 2913(m), 1638(s), 1478(m), 1143(m), 957(w), 358(w). ESI-TOF MS: m/z
(%); [M – 3H]+ 773.19. UV-Vis (CHCl3, λmax, nm), 300, 317 (shoulder), 449. Anal. calcd. for
C40CuH52N4O2S4: C, 59.12; H, 6.45; N, 6.89; S, 15.78. Found: C, 59.80; H, 6.08; N, 6.81; S,
15.85.
3.3.10 Synthesis of [Cu-(DL4)2].3H2O (10)
The reaction of DL4 (0.2 g, 0.50 mmol) and CuCl2.2H2O (0.04 g, 0.25 mmol) in acetonitrile
furnished complex 10 as a deep brown powder. Yield 89 %. Decomposition tempt. range, 268–
270 °C. ESI-TOF MS: m/z (%); [M]+. 798.99. IR υ (cm-1) 2973(w), 1641(s), 1471(m),
1125(m), 883(m).370(w). UV-Vis (CHCl3, λmax, nm), 301, 321 (shoulder), 449. Anal. calcd.
for C32Cl4CuH34N4O3S4: C, 44.89; H, 4.00; N, 6.54; S, 14.98. Found: C, 44.60; H, 3.87; N,
6.29; S, 15.05.
3.3.11 Synthesis of [Cu-(DL5)2].3H2O (11)
The reaction of DL5 (0.20 g, 0.40 mmol) and CuCl2.2H2O (0.40 g 0.20 mmol) in acetonitrile
furnished complex 11 as a reddish-brown powder. Yield 84 %. Decomposition tempt. range,
285 – 287 °C. IR υ (cm-1) 2964(w), 1640(s), 1434(m), 1130(m), 885(s).390(w). ESI-TOF MS:
m/z (%); [M]+.911.13. UV-Vis (CHCl3, λmax, nm), 301, 323 (shoulder), 449. Anal. calcd. for
C40Cl4CuH50N4O3S4: C, 49.61; H, 5.20; N, 5.79; S, 13.24. Found: C, 49.44; H, 4.94; N, 5.61;
S, 12.80.
10

3.3.12 Synthesis of [Cu-DL6)2] (12)
The reaction of DL6 (0.20 g, 0.50 mmol) and CuCl2.2H2O (0.04 g, 0.25 mmol) in acetonitrile
furnished complex 12 as a deep brown powder. Yield 86 %. Decomposition tempt. range, 274
– 276 °C. ESI-TOF MS: m/z (%); [M – H]+, 827.03. IR υ (cm-1) 2971(w), 1639(s), 1436(m),
1125(m), 883(s).354(w). UV-Vis (CHCl3, λmax, nm), 300, 321 (shoulder), 449. Anal. calcd for
C34Cl4 CuH32N4S4: C, 49.19; H, 3.89; N, 6.75; S, 15.45. Found: C, 48.89; H, 3.60; N, 6.59; S,
15.26.
3.4
Single-crystal X-ray diffraction
Crystal evaluation and data collection was done on a Bruker Smart APEXII diffractometer with
Mo Kα radiation (I = 0.71073 Å) equipped with an Oxford Cryostream low temperature
apparatus operating at 100K for all samples. Reflections were collected at different starting
angles and the APEXII program suite was used to index the reflections [22]. Data reduction
was performed using the SAINT [23] software and the scaling and absorption corrections were
applied using the SADABS [24] multi-scan technique. The structures were solved by the direct
method using the SHELXS program and refined using SHELXL program [25]. Graphics of the
crystal structures were drawn using Mercury software [26]. Non-hydrogen atoms were first
refined isotropically and then by anisotropic refinement with the full-matrix least square
method based on F2 using SHELXL. In complex 1, solvent mask was calculated and 48.7
electrons was found in a void with a volume of 203.0 Å3 in 1, consistent with the presence of
1[C6H5CH3] per formula unit which account for 48.7 electrons. The solvent molecule was
toluene in complex 1 and was highly disordered. Attempts to model it was unsuccessful and
only led to unstable refinement and was therefore omitted using SQUEZE [27] option in
PLATON [28]. The carbon atom of the DCM molecule in complex 5 was found to be disordered
over 2 positions with the major component having 53.80 % site occupancy while toluene
molecules in compound 11 was found to be disordered over an inversion centre and was
modeled using PART -1 instruction with fixed site occupancy factor of 0.5. The
crystallographic data and structure refinement parameters for complex 1, 2, 5 and 11 are given
in Table 1.
11

Table 1: The summary of X-ray crystal data collection and structure refinement parameters for complex 1, 5, 8, and 11
1
5
8
11
Empirical formula
Formula weight
Crystal system
Space group
C40H42Cl4N4NiS4.2CH2Cl2
1077.38
C40H42Cl4CuN4S4.C6H5.CH3
C36 H38 N4 Ni S4
713.65
C52 H70 Cu N4 S4
1004.49
monoclinic
P21/c
942.9
Triclinic
triclinic
Monoclinic
P21/n
P -1
P-1
9.4674(16)
12.120(2)
12.249(2)
75.343(5)
68.614(6)
77.373
12.3469(6)
14.0658(8)
14.6828(8)
90
a/Å
b/Å
c/Å
α/°
β/°
γ/°
Volume/Å3
8.55440(10)
11.2627(2)
13.0340(2)
105.0640(10)
100.016(2)
94.213(3)
1184.80(3)
1
12.5278(3)
12.3052(3)
14.7214(4)
90
109.835(3)
90
114.8480(10),
90
1253.6(4)
1
2398.7(2)
2
2059.32(9)
2
Z
ρcalcg/cm3
1.51
1.391
1.151
1.249
μ/mm-1
1.073
0.89
0.701
0.64
F(000)
554
1042
748
503
Crystal size/mm3
2Θ range for data collection/°
Index ranges
0.4 × 0.27 × 0.14
3.302 to 54
-10 ≤ h ≤ 8
-14 ≤ k ≤ 14
-15 ≤ l ≤ 16
0.21×0.18×0.08
1.753 to 27.543
-16 ≤ h ≤ 16
-18 ≤ k ≤ 16
-19 ≤ l ≤ 18
0.190 x 0.120 x 0.080
1.799 to 27.563
16 ≤ h ≤ 16
-14 ≤ k ≤ 14
-15 ≤ l ≤ 16
27584
0.210 x 0.150 x 0.080
1.754 to 25.498
-18 ≤ k ≤ 16
-18 ≤ k ≤ 16
-19 ≤ l ≤ 18
Reflections collected
16602
29811
8942
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
5049 [Rint = 0.0151]
5049/1/282
5222 [Rint = 0.0186]
5478/123/309
1.029
4729 [R(int) = 0.0421]
4729 / 0 / 205
1.021
4564 [R(int) = 0.0194]
4564 / 1 / 285
1.043
1.029
Final R indexes [I>=2σ (I)]
Final R indexes [all data]
Largest diff. peak and hole (e Å-3 )
Solvent for growing crystals
R1 = 0.0328, wR2 = 0.0802
R1 = 0.0388, wR2 = 0.0835
0.97 and -1.10
R1 = 0.0500, wR2 = 0.1080
R1 = 0.0500, wR2 = 0.1240
0.789 and -0.647
Toluene
R1 = 0.0359, wR2 = 0.0860
R1 = 0.0552, wR2 = 0.0929
0.455 and -0.315
Toluene
R1 = 0.0820, wR2 = 0.2009
R1 = 0.1027, wR2 = 0.2172
4.469 and -0.753
Toluene
Dichloromethane
12

3.5
Determination of free radical scavenging activity
DPPH (1,1-diphenyl-2-picrylhydrazyl) radical scavenging assay of compound 1–12 was carried
out as per reported method by Chandrika et al with slight modifications [29]. Briefly, 100 µL of
varying concentration (1.0 mM, 0.75 mM, 0.50 mM, and 0.25 mM) of the test sample was added
to equal quantity of 0.1 mM solution of DPPH in ethanol. The reaction mixture was vortexed
carefully and left in the dark at room temperature for 30 min. After 30 min of incubation at room
temperature, the DPPH reduction was measured by reading the absorbance at 517 nm. Ascorbic
acid of varying concentration (1.0 mM, 0.75 mM, 0.50 mM, and 0.25 mM) was used as the
reference compound. The ability of the compounds to scavenge DPPH radical was calculated as:
퐴푏푠표푟푏푎푛푐푒 푐표푛푡푟표푙 ‒ 퐴푏푠표푟푏푎푛푐푒 표푓 푠푎푚푝푙푒 푥 100
=
% Scavenging Activity
퐴푏푠표푟푏푎푛푐푒 푐표푛푡푟표푙
3.6
In vitro antimicrobial studies
The antimicrobial activity of the Ni(II) and Cu(II) dithiocarbamate metal complexes was evaluated
against four gram-negative bacteria, viz: Salmonella typhimurium ATCC 14026, Pseudomonas
aeruginosa ATCC 27853, Escherichia coli ATCC 25922 and Klebsiella pneumoniae ATCC 31488
and two gram-positive bacteria, viz: Staphylococcus aureus ATCC 700699 (methicillin resistant)
and Staphylococcus aureus ATCC 25923. Ciprofloxacin was used as a standard antibiotic for
comparison while DMSO was used as a negative control and it showed no antibacterial activity
against any of the bacterial strains used for this study at different concentrations. The samples
were prepared by dissolving 1000 µg of test sample in 1 ml of dimethyl sulfoxide (DMSO). The
bacteria were inoculated onto Nutrient Agar (NA) (Biolab, South Africa) plates using the streak
plate technique and incubated at 37 °C for 18 hr [30]. A single colony was isolated and inoculated
into 10 ml sterile Nutrient Broth (NB) (Biolab, South Africa). This was incubated at 37oC for 18
hr in a shaking incubator (100 rpm). The concentration of each bacterial strain was adjusted with
sterile distilled water to achieve a final concentration equivalent to 0.5 Mc Farland’s Standard (i.e
1.5 x 108 cfu/mL) using a densitometer (Mc Farland Latvia) [31]. Thereafter, the MHA plates were
lawn inoculated with the diluted bacteria using a sterile throat swab. 5 µL of each sample was
spotted onto the MHA plates and the plates were incubated at 37oC for 18 hr and then assessed for
antibacterial activity which was denoted by a clear zone at the point of spotting. Samples that
showed antimicrobial potential during antibacterial screening were tested further to determine their
13

minimum inhibitory concentration (MICs). The samples were serially diluted 10 times to achieve
concentrations ranging from 1000 µg/ml to 0.2 µg/ml. For the samples where MICs were lower
than 0.2 µg/mL, the solutions were further diluted serially 5 times to achieve concentrations
ranging from 0.100 µg/ml to 0.00625 µg/ml. 5µl of each sample at different concentrations was
spotted onto the MHA plates and the plates were incubated at 37 °C for 18 hr and then assessed
for their MIC. These were done in triplicate and the MIC was determined as the lowest
concentration of the compounds at which no visible bacterial growth was observed after
incubation.
Results and discussion
4.0
Synthesis of N,N'-diarylformamidines dithiocarbamate ligands and their Cu (II)
and Ni (II) metal complexes.
The synthesis routes for DL1 - DL6 and complexes 1 - 12 are shown in Scheme 1. Synthesis of
the potassium dithiocarbamate salts, DL1 – DL6 was achieved by the reaction of equimolar
amounts of the appropriate formamidine with carbon disulfide in the presence of potassium
hydroxide in good yields between 74 and 94 % and with melting points between 237 – 263 °C.
Complexes 1 - 12 were obtained by the reaction of salts of potassium dithiocarbamates with CuCl2
or NiCl2 in a 2:1 ratio as air stable purple or red solids for Ni(II) complexes and brown solids for
Cu(II) complexes respectively. The Ni(II) complexes generally had lower decomposition ranges
(between 265 – 282 °C) compared to the Cu(II) complexes (269 – 296 °C). The decomposition
temperatures were dependent on the substituents on the backbone of the dithiocarbamates and was
observed to be higher in those with symmetrical backbones relative to the unsymmetrical form of
the complexes. All complexes showed good solubility in chloroform, dichloromethane and toluene
but were only partially soluble in other polar solvents.
14

Scheme 1: Synthesis of symmetrical and unsymmetrical N,N'-diarylformamidine dithiocarbamate
ligands DL1 – DL6 and their Ni(II) and Cu(II) metal complexes 1-12.
4.1
Spectroscopic studies
Nuclear magnetic resonance
The 1H NMR data of DL1 – DL6 were obtained in DMSO and peak assignments done using 2D
NMR. The azomethine (NC(H)=N) and amine protons were used to follow the transformation
from formamidines to potassium dithiocarbamates to the metal complexes. The disappearance of
the amine proton, which appears in the region 5.43 – 5.60 in L1 – L6 marked successful synthesis
of DL1 – DL6. The azomethine proton was also observed to shift from 7.25 – 8.69 in L1 – L6 to
9.86 – 10.39 in DL1 – DL6. These further shifted upfield on complexation to the metal salts in
complexes 1 – 6 and appeared in the region 8.81 – 9.10 (Table 2). There were also typical aliphatic
signal shifts in moving from the formamidines to the dithiocarbamates to complexes all influenced
by the electronics on the formamidine backbone and complexation. Upon coordination to Ni(II),
the methyl group shifted downfield in all the Ni (II) complexes, (1-6). This slight downfield shift
in 1 - 6 implies the drift of electron cloud towards the metal ion center [32, 33]. In addition, the
13C-NMR spectra of 1 - 3 show that the quaternary thiouride (-NCS2) carbon peaks shifted upfield
on complexation of DL1 – DL3. This upfield shift is attributed to the delocalization of an electron
15

cloud from the –NCS2 moiety towards the metal centre, hence lowering the C=S bond strength [34,
35].
Table 2: The -NCS2 (13C-NMR) and NC(H)=N (1H-NMR) signals for DTL1 – DTL6 and 1 – 6,
and the IR bands of thiouride (C—N) and azomethine (C=N) for ligands and the complexes,
Ligands
δ (-NCS2) ppm
Δ δ
δ NC(H)=N
Δ δ
υ(C=N) cm-1
Δυ
υ(C—N) cm-1
Δυ
(Complex)
ppm
DTL1 (1)
DTL2 (2)
DTL3 (3)
DTL4 (4)
DTL5 (5)
DTL6 (6)
DTL1 (7)
DTL2 (8)
DTL3 (9)
DTL4 (10)
DTL5 (11)
DTL6 (12)
217.62 (216.95)
220.94 (206.98)
218.95 (217.92)
218.82 (219.32)
217.02 (218.76)
219.04 (219.49)
0.69
6.13
1.84
0.50
1.74
0.45
9.86(8.85)
10.15(8.88)
9.92(8.81)
10.12(8.94)
10.13(9.10)
10.39(8.91)
1.01
1.27
1.11
1.18
1.03
1.48
1640 (1647)
1639 (1651)
1629 (1644)
1614 (1642)
1603 (1639)
1612 (1644)
1640 (1645)
1639 (1663)
1629 (1638)
1614 (1641)
1603 (1640)
1612 (1639)
7
1467 (1472)
1452 (1469)
1477 (1479)
1432 (1474)
1430 (1469)
1435 (1468)
1467 (1470)
1452 (1477)
1477 (1478)
1432 (1471)
1430 (1434)
1435 (1436)
7
8
12
15
28
36
32
5
1
3
5
1
3
24
9
25
1
27
37
27
39
4
1
Fourier transform infrared spectroscopy
Four major vibrational bands were observed in the IR spectra of the dithiocarbamate ligands (DL1
– DL6). The stretching bands observed were for υ(C=N) [1603 – 1640] of azomethine (C(H)=N),
the υ(C—N) [1430 – 1477] of thiouride, υ(C=S)str [1000 – 1152] and the υ(C—S) [921 – 1036]
Table 2. Upon complexation, these bands all moved to higher frequencies; the υ(C=N) of
azomethine appeared between 1639 and 1651 cm-1 from 1603 and 1640 cm-1 for the
dithiocarbamate ligands. The band of C—Nstr appeared between 1435 and 1478 cm-1 in 1 – 6 from
1430 - 1467 cm-1 in DL1 – DL6 (Table 3). The shifts could be because of delocalization of electron
densities of the ligand towards the metal centres resulting in partial double bond character of the
υ(C—N) bond [7, 10]. The frequency of the C—N band in this complexes lies between that of a
16

typical C=N and that of typical C—N bond (υ(C=N) = 1640–1690 cm-1 while υ(C—N) = 1250–
1360 cm-1) [19, 36, 37]. The υ(C—S) bands appeared as a single band in the region of 886-1096
cm-1 in all complexes alluding to symmetrical bonding of the dithiocarbamate ligand to the metal
center in the complexes, supporting bidentate coordination [37-39]. The spectra of the 1 - 12 also
show a band in the far infra-red region between 350 and 384 cm-1 assigned to the υ(M—S) bonds
[40].
UV-vis spectroscopy
The electronic absorption spectra of the dithiocarbamate ligands and the metal complexes were
recorded in DMSO and chloroform respectively. The UV-Visible spectra of all the ligands and
twelve complexes are given in Figures (1a – 1c). Generally, transition due to the ligands appeared
in the UV region while the d–d transition appeared in the visible region [11]. The spectra of the
ligands DL1 – DL6 showed two absorption bands in the UV region between 289 and 300 nm and
338 and 345 nm. These absorption bands are assigned to intraligand ℼ→ℼ* associated to N-C=S
group and ℼ→ℼ* transition within the S-C=S group [41]. In each of the spectra, three absorption
bands for 1 – 6 between 258 and 263 nm, 340 and 343 nm, and 454 and 467 nm are observed. The
bands between 454 and 467 can be attributed to d-d transition while those below 400nm are
assigned to intra-ligand ℼ→ℼ* associated to N-C=S and S-C=S group of the dithiocarbamate
ligand backbone. For 7 – 12, two major absorption bands between 300 and 301 nm and 442 ad 449
nm are observed.. Highly-intense bands were observed within the range of 442 – 449 nm and these
bands are assigned to d-d transitions [42]. There is a possibility of three observable bands in the
spectra of 7 – 12 as shoulder is observed in each alluding an overlap of two bands.
17

Figure (1a): Electronic absorption spectra of DL1 – DL6
(1c): Electronic absorption spectra of 7 – 12.
(1b): Electronic absorption spectra of 1 - 6
4.2
X-ray structural analysis
Suitable crystals for single crystal X-ray diffraction analysis were obtained for complexes 1, 5, 8
and 11 by slow evaporation where, crystals of 1, 8 and 11 were obtained from their solution of
toluene, while those of 5 were obtained from a dichloromethane solution of it. The molecular
structures are given in Figures 2. And selected bond distances and angles in Table 3. Complex 1
and 8 contain half a molecule of the complex in their asymmetric units while 5 and 11 contain half
a molecule together with toluene molecule. The structures of all the complexes consist of a
mononuclear neutral species in which the metal centers [Ni(II) and Cu(II)] are coordinated by two
pairs of sulphur atoms from two bidentate dithiocarbamate ligand in a distorted square planar
geometry fashion around the metal center. The S—M—S bond angles are 79.441(18), 79.402(18),
76.77(5) and 77.333(3)° smaller than the ideal 90° for square planar geometry) for 1, 5, 8 and 11
respectively. The strained S—C—S angles of all the complexes contribute to the deviation from
ideal square planar geometry. In the structures of 1 and 5, the Ni—S bond distances range from
2.2003(5) to 2.2090(5) while in 8 and 11 the Cu—S bond distance ranges between 2.2597(15) and
2.3282(7) as seen in Table 4. These values are consistent with the values reported with
mononuclear bis dithiocarbamate complexes [17]. The M—S bond distances seem not to be
influenced much by electronics of the CS2 backbone. As expected, the Cu—S bond at the metal
center of 8 and 11 are greater than the Ni—S bond of complexes 1 and 5. The C—S bond distances
seem not to favor single or double bond character in all the named complexes. They are shorter
than the typical single C—S bond length of 1.82 Å but longer than the C=S double bond length of
1.67 Å [43]. This is an indication of delocalization of electron along the S—C—S moiety in the
complexes. [44]. Also, the C—N bond length of the -NCS2 moiety of the complexes deviates from
the reported value for single C—N which is 1.47Å. For complex 1, C(10)—(N)2 = 1.344(2),
C(1)—N(2) = 1.358(2) for complex 5, C(1W)—N(1) = 1.344(7) for complex 8 and C(10)—(N)2
= 1.362(4) for complex 11. This indicates the delocalization of ℼ-electron over the entire S2CN
fragment in the complexes [42].
18

Figure 2: ORTEP diagram of complex 1, 5, 8, and 11 drawn at 50 % thermal ellipsoids probability.
Hydrogen atoms have been omitted for clarity.
Table 3: Selected bond length (Å) and angles (°) for complex 1, 5, 8 and 11
Parameters
Bond distances
M—S
1
5
8
11
2.2003(5)
2.2090(5)
1.698(2)
1.711(2)
1.344(2)
2.2023(5)
2.2045(3)
1.696(2)
1.709(2)
1.358(2)
2.2597(15)
2.3057(13)
1.695(5)
2.2643(8)
2.3282(7)
1.688(3)
1.706(3)
1.362(4)
M—S
C—S
C—S
1.731(5)
C—N
1.344(7)
Bond angles
S—M—S
S—C—S
79.4421(18)
111.50(11)
79.402(18)
111.53(11)
76.77(5)
77.33(3)
111.70(3)
115.47(17)
4.3
Antimicrobial activities evaluation
19

When designing new compounds, its ideal to have them as an excellent antimicrobial agent capable
of killing pathogens without having any negative side effects when used for treatment [36].
Complexes 1 - 12 and along with Ciprofloxacin as a standard were screened against six bacteria
strains and the results are presented in Table 4. The results based on the MICs obtained show that
the complexes possess good antimicrobial activities against five bacterial strains with the
exception of MRSA.
Complexes 1, 4, 5, 7 and 10 showed better activity against E. coli and even better than the standard
ciprofloxacin. The antimicrobial activities of complexes with the symmetrically substituted
formamidines, 1 - 3 and 7 – 9 were more active compared to those with unsymmetrically
substituted formamidine 4 - 6 and 10 - 12 (form). For example, the MICs of 1, 2 and 3 were 0.10,
6.25 and 1.6 ug/ml respectively while those of 4, 5 and 6 were 0.025, 0.10 and 0.20 ug/ml
respectively when tested against E. coli (Table 4). Complexes in which the formamidine bore an
electron withdrawing group (Cl in this instance) 4 - 6 and 10 - 12 were more active and activity
can be attributed to better lipophilicity , hence easy penetration into the lipophilic section of the
cell membrane or lipophilic domains of proteins. [45]. This trend was observed with the other
bacterial strains as well.
While most complexes displayed moderate activities against S. typhimurium, 6 and 10 showed
higher activities compared to the other complexes and ciprofloxacin. It could be observed that
there was varying degrees of activity of all the complexes against E. coli, S. typhimurium and
other bacterial strains. This could be due to the nature of the metal ion electronic configuration of
the complexes as well as the nature of the cell membrane of each bacteria strain which determines
the permeability of the complexes or the difference in ribosome of the microbial cell [46, 47].
Complex 7 exhibited least activity against P. aeruginosa, whilst complexes 8, 11 and 12 displayed
better activity as compared to ciprofloxacin. Complex 11 displayed the highest activity against P.
aeruginosa with an MIC value of 0.20 µg/ml. All the complexes showed better activity against K.
pneumonia relative to ciprofloxacin with the exception of complexes 1, 7, 9, 10 and 12. However,
all the complexes were inactive against MRSA apart from complexes 4, 5 and 6 which were active
only at the highest concentration (1000 µg/mL). The failure of the complexes to inhibit MRSA
could be as a result of the inability of the compounds to penetrate through the bacteria cell or the
complexes might have been modified or rendered inactive as they entered the cell wall of the
20

MRSA [48]. All the Cu(II) complexes showed better activity against S. aureus excluding complex
11 while all Ni(II) complexes displayed moderate to low activity against it.
Table 4: Minimum inhibitory concentration of the metal complexes (µg/mL).
Gram (-) bacteria
S. typhimurium P. aeruginosa
Gram (+) bacteria
Complexes
E. coli
K. pneumoniae S. aureus MRSA
Ni (II) and Cu (II) complexes of symmetrical N,N'-diarylformamidine dithiocarbamate
1
2
3
7
8
9
0.10
6.25
1.6
100
100
50
50
6.25
0.20
1000
1000
1000
0.80
1.60
0.80
NA
NA
NA
NA
NA
NA
6.25
1.60
100
0.40
12.5
0.20
0.05
100
12.5
0.80
0.40
1.60
6.25
0.00625
3.125
Ni(II) and Cu(II) complexes of unsymmetrical N,N'-diarylformamidine dithiocarbamate
4
0.025
0.10
12.5
1.6
1.60
6.25
3.125
1.60
0.20
0.40
0.80
0.20
0.20
1000
6.25
50
1000
1000
1000
NA
5
6
0.20
0.2
0.10
10
0.025
3.125
0.80
0.20
0.40
1.60
0.40
3.125
0.00625
1.60
1.60
1000
0.025
25
11
12
NA
NA
Ciprofloxacina
0.20
1.60
25
NA = No activity, a = standard
4.4 DPPH Radical scavenging ability
21

2,2-Diphenyl-1-picrylhydrazyl (DPPH) is a stable free radical containing an odd electron in its
structure [49]. It is usually used for the evaluation of radical scavenging activity in a relatively
shorter time compared to other methods [50]. The ability of antioxidant molecules to scavenge the
radical of the DPPH is due to their hydrogen or electron radical donating capability [51]. We have
used DPPH radical scavenging assay for the screening of the antioxidant activity of the Ni(II) and
Cu(II) dithiocarbamate metal complexes at different concentrations. The antioxidant activity of
dithiocarbamates as well as their metal complexes may be elucidated on the basis of electron
donating ability of sulfur and the central metal ions in their various complexes which led to free
radical stabilization [52]. It has been reported that the presence of the central metal increases the
antioxidant activity of the ligand since the ligands proton donor capacity is enhanced [4, 53]. The
DPPH is purple in color and the odd electron in its radical gives a strong absorption maximum at
517 nm. The color turns from purple to yellow when the odd electron of the DPPH radical becomes
paired with hydrogen from free radical scavenging antioxidants to form reduced DPP-H [6, 54].
The % free radical scavenging ability values were used to calculate the IC50 values of complexes
1-12 and these are summarized in Table 6. Results were compared with the antioxidant activity of
ascorbic acid (with IC50 value of 1.01 x 10-3 mM). The IC50 values were used to determine the
antioxidant activity i.e. the lower the IC50 value the higher will be the antioxidant activity [55,
56]. We found out that complex 2 have the least IC50 value of 1.10 x 10-3 mM and has the highest
value of ascorbic acid equivalent antioxidant capacity (AEAC), followed by 7, 8, 3 and 1
respectively as shown in Table 4. On the other hand, complex 6, 10 and 12 displayed weak
antioxidant activity. The radical scavenging activities of complexes 1-3 and 7-9 which have rich
electron seems to be better than the activities of the complexes 4-6 and 10-12 with electron
withdrawing group. For example, complexes 1, 2 and 3 have higher activities compared to 4, 5
and 6 as seen in Table 3. A trend was observed for complexes 6-9 to 10-12. It has been shown
previously [57] that electronics play a crucial role in enhancing free radical scavenging activity.
In 1-3 and 6-9 the presence of electron donating group increases the electron density at the carbon
atoms in the aromatic rings of the complexes, hence, increasing their electron donating capability
which lead to their higher antioxidant activity relative to 4-6 and 10-12 that have electron
withdrawing groups. Generally, the antioxidant activity of 1-12 increases as the concentration
increases and this is illustrated in Fig 3 and 4.
Table 5 :
Antioxidant potential of tested compounds at different concentration.
22

Complexes
IC50 (mM)
6.59 x 10-3
1.10 x 10-3
2.99 x 10-3
7.12 x 10-2
1.21
1
2
3
4
5
6
1.85
7
1.65 x 10-3
4.50 x 10-3
1.60 x 10-1
2.16
8
9
10
11
12
1.383
2.91
Ascorbic acid
1.04 x 10-3
Result presented here are the mean values from three independent experiments
120.00
90.00
60.00
30.00
0.00
0.25mM
Ni-DL1
0.5mM
0.75mM
1mM
Concentration (mM)
Ni-DL2
Ni-DL3
Ni-DL4
23

Figure 3: % Free radical scavenging vs concentration (mM) of Ni((II) dithiocarbamate metal
complexes (1—6)
90.00
60.00
30.00
0.00
0.25mM
Cu-DL2
0.5mM
0.75mM
1mM
Concentration (mM)
Cu-DL1
Cu-DL3
Cu-DL4
Cu-DL5
Cu-DL6
ASC ACID
Figure 4: % Free radical scavenging vs concentration (mM) of Cu(II) dithiocarbamate metal
complexes (7 - 12).
5.0
Conlusion
Ni(II) and Cu(II) complexes of symmetrical and unsymmetrical formamidine-based
dithiocarbamate ligands were synthesized and fully characterized by UV-Visible, FT-IR, NMR
and Mass spectrometry. X-ray structural analysis of 1, 5, 8 and 11 showed that the central metal,
Cu(II) and Ni(II) are bonded by four sulfur atoms from the two N,N'-diarylformamidine-based
dithiocarbamate ligands in a bidentate coordination mode to form a slightly distorted square planar
geometry. The results of their antioxidant studies showed that, complexes with symmetrical
formamidine-based dithiocarbamate ligands exhibited higher antioxidant activity compared to
their unsymmetrical counterparts with complex 2 having the highest ascorbic acid equivalent
capacity. The in vitro antibacterial screening and the MICs showed that, complexes 1-12 had
moderate to good antimicrobial activities against all the bacterial strains except MRSA. The
presence of chlorine atoms in complexes with unsymmetrical formamidine-based dithiocarbamate
24

ligands enhanced their antibacterial activity, thus showing better activities compared to their
symmetrical counterparts.
Conflict of interest
The authors declare no competing financial interest
Acknowledgements
The authors would like to thank the college of Agriculture, Science and Engineering, University
of Kwazulu-Natal and the National Research Foundation (NRF), South Africa for financial
support.
ORCID
Bernard Omondi http://orcid.org/0000-0002-3003-6712
Supplementary data
CCDC 1917370, CCDC 1917371, CCDC 1917372 and CCDC 1917373 contain the supplementary
crystallographic data for complexes 1, 5, 8 and 11 respectively. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44)1223-336-
033; or via e-mail: deposit@ccdc.cam.ac.uk.
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Synthesis and structural studies of nickel(II)- and copper(II)-N,N'-diarylformamidine
dithiocarbamate complexes as antimicrobial and antioxidant agents
Segun D. Oladipo,a Bernard Omondi*b and Chunderika Mocktar c
a School of Chemistry and Physics, Westville Campus, University of Kwazulu-Natal, Private Bag
X54001, Durban, 4000, South Africa.
b
School of Chemistry and Physics, Pietermaritzburg Campus, University of Kwazulu-Natal,
Private Bag X01, Scottsville, 3209, South Africa.
c Discipline of Pharmaceutical Sciences, School of Health Sciences, University of Kwazulu-Natal,
Private Bag X54001, Durban, 4000, South Africa.
*Corresponding author
E-mail: owaga@ukzn.ac.za Tel: (+27) 332606436
Graphical Abstracts
28

Antioxidant using
DPPH assay
Antibacterial activity
evaluation
Synthesis and structural studies of nickel(II)- and copper(II)-N,N'-diarylformamidine
dithiocarbamate complexes as antimicrobial and antioxidant agents
Segun D. Oladipo,a Bernard Omondi*b and Chunderika Mocktar c
a School of Chemistry and Physics, Westville Campus, University of Kwazulu-Natal, Private Bag
X54001, Durban, 4000, South Africa.
b
School of Chemistry and Physics, Pietermaritzburg Campus, University of Kwazulu-Natal,
Private Bag X01, Scottsville, 3209, South Africa.
c Discipline of Pharmaceutical Sciences, School of Health Sciences, University of Kwazulu-Natal,
Private Bag X54001, Durban, 4000, South Africa.
*Corresponding author
E-mail: owaga@ukzn.ac.za Tel: (+27) 332606436
29