Synthesis, characterization, thermal, antimicrobial and antioxidant studies of some transition metal dithiocarbamates

Article abstract of DOI:10.1007/s11164-016-2709-2

Metal dithiocarbamate complexes of Co(II) [1], Cu(II) [2], Mn(II) [3], Cr(III) [4], and Pd(II) [5] have been synthesized using sodium N-ethyl-N-phenyldithiocarbamate (NaL). The complexes were characterized by elemental analyses, FTIR and UV–vis spectrosco

Full text of DOI:10.1007/s11164-016-2709-2

Res Chem Intermed
DOI 10.1007/s11164-016-2709-2
Synthesis, characterization, thermal, antimicrobial
and antioxidant studies of some transition metal
dithiocarbamates
Damian C. Onwudiwe1,2 Anthony C. Ekennia3
•
Received: 20 May 2016 / Accepted: 24 August 2016
Ó Springer Science+Business Media Dordrecht 2016
Abstract Metal dithiocarbamate complexes of Co(II) [1], Cu(II) [2], Mn(II) [3],
Cr(III) [4], and Pd(II) [5] have been synthesized using sodium N-ethyl-N-
phenyldithiocarbamate (NaL). The complexes were characterized by elemental
analyses, FTIR and UV–vis spectroscopic techniques, magnetic moment, molar
conductance and thermal analyses (TGA and DSC). The infrared spectra indicated
the coordination of dithiocarbamate through the two sulphur atoms in a symmetrical
bidentate fashion. The thermal behavior of these complexes showed that the
hydrated complexes lost water molecules in the first step, followed by decompo-
sition of the ligand molecules in the final steps. Mass loss considerations at these
final decomposition steps indicate conversion of the complexes to sulphides. The
antimicrobial potentials of the complexes were evaluated against some selected
bacteria strains (Escherichia coli, Pseudomonas aureginosa, Salmonella typhi and
Staphylococcus aureus) and fungi organisms (Aspergillus flavus and Fasiparium
oxysporium). The compounds showed a broad spectrum of fungicidal and bacteri-
cidal activities which exceeds that of the control drugs at a 100 lg/mL concen-
tration. The antioxidant properties of the ligand and its Cu(II) complex were
evaluated in vitro using DPPH assay, and the complex was found to exhibit better
radical scavenging ability than the free ligand.
&
Damian C. Onwudiwe
Damian.Onwudiwe@nwu.ac.za
1
Material Science Innovation and Modelling (MaSIM) Research Focus Area, Faculty of
Agriculture, Science and Technology, North-West University (Mafikeng Campus),
Private Bag X2046, Mmabatho, South Africa
2
3
Department of Chemistry, School of Mathematical and Physical Sciences, Faculty of
Agriculture, Science and Technology, North-West University (Mafikeng Campus),
Private Bag X2046, Mmabatho 2735, South Africa
Department of Chemistry, Federal University, Ndufu-Alike Ikwo (FUNAI),
P.M.B. 1010, Abakaliki, Ebonyi State, Nigeria
123

D. C. Onwudiwe, A. C. Ekennia
Keywords Transition metals Á Dithiocarbamate Á Thermal studies Á Antimicrobial Á
Radical scavenging
Introduction
In the past two decades, the number of invasive and systemic pathogenic infections
has increased dramatically [1] due to the growing number of immunocompromised
patients as a result of cancer chemotherapy, organ transplantation and HIV infection
[2, 3]. Consequently, drug resistance cases have become a growing concern in the
treatment of infectious diseases caused by bacteria and fungi [4] organisms. For
example, penicillin- and methicillin-resistant strains of Staphylococcus aureus
(MRSA) are common in hospitals and are emerging in communities around the
world [5]. Hence, the discovery and development of effective antibacterial and
antifungal drugs with novel mechanisms of action have become urgent tasks for
infectious disease research programs [6].
On the other hand, reactive oxygen species (ROS), such as the superoxide radical
anion (O-2 Á), hydroxyl radical (OHÁ) and hydrogen peroxide (H2O2) are formed in
biological systems by the partial reduction of molecular oxygen [7]. Overproduction
of ROS is considered to be the main cause of oxidative damage of biomolecules
such as DNA, lipids and proteins, thereby accelerating cancer, ageing, inflamma-
tion, cardiovascular and neurodegenerative diseases [8]. Consequently, antioxidant-
based research on synthetic and natural compounds has become one of the major
areas of scientific research [9] that is channeled toward their ability to protect
organisms and cells from damage induced by oxidative stress [10, 11]. The potential
value of antioxidants has motivated scientists to search for the cooperative effects of
compounds on improving antioxidant activity and cytotoxicity [12]. The DPPH
(2,20-diphenyl-1-picrylhydrazyl) assay is one of the most important and widely
accepted methods used for the in vitro evaluation of antioxidant activity of both
natural and synthetic compounds. It is technically a simple and rapid method for
antioxidant screening [13]. DPPH radicals may be neutralized either by direct
reduction via electron transfer or by radical quenching via proton transfer [14] and
can be determined spectrophotometrically, or by high-performance liquid chro-
matography, or by using some faster and inexpensive methods like thin layer
chromatography [13].
Most synthetic compounds have been reported to exhibit antioxidant properties
[15, 16], however reports on the antioxidant potentials of dithiocarbamate
compounds are scare, even though dithiocarbamate (DTC) moiety has been
identified in a number of biologically active molecules [17, 18]. Our group has been
interested in the biological properties of metal complexes of N-alkyl-N-
phenyldithiocarbamates [17, 19, 20] and their usefulness as sources of metal sulfide
deposition. Also, there have been numerous reports on the biological potentials of
dithiocarbamate complexes [21–23]; hence, our interest in complexes of N-alkyl-N-
phenyldithiocarbamates. Herein, we report the synthesis, spectroscopic, thermal and
biological (antimicrobial and radical scavenging) studies of Co(II), Cu(II), Mn(II),
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Synthesis, characterization, thermal, antimicrobial and…
Cr(III), and Pd(II) complexes of N-ethyl-N-phenyldithiocarbamate. The compounds
were screened against Gram-negative strains (Escherichia coli and Salmonella
typhi) and Gram-positive bacteria strains (Pseudomonas aureginosa and S. aureus)
and fungi organisms (Aspergillus flavus and Fasiparium oxysporium). The microbial
strains were chosen based on their clinical and pharmacological importance [24]. In
addition, thermal studies of the compounds were conducted in order to investigate
the potentials of the complexes as single-source precursors in metal sulphide
depositions [25].
Experimental
General
All reagents and solvents were commercially available high-grade materials
(Merck/Sigma-Aldrich) and were used as received. The solvents were purified by
standard methods [26].
Physical measurements
Elemental analysis was performed on an Elementar vario EL Cube set up for
carbon, hydrogen, nitrogen and sulfur (CHNS) analysis. Infrared (IR) spectra were
recorded on a Bruker alpha-P Fourier transform IR (FTIR) spectrometer in the
frequency range 4000–500 cm-1. The electronic spectra of the metal complexes
were recorded on a Perkin-Elmer k20 ultraviolet–visible light (UV–Vis) spec-
trophotometer in chloroform. Room-temperature magnetic moments and molar
conductance measurements were recorded on a Johnson Matthey magnetic
susceptibility balance and a MC-1 Mark V conductivity meter with a cell constant
of 1.0, respectively. The thermal behaviour of the complexes was studied using the
simultaneous thermal analysis (STA) technique for parallel recording of TG
(thermogravimetry) and differential scanning calorimetry (DSC) curves on an
SDTQ 600 Thermal instrument. Samples were contained within alumina crucibles
and heated at 10 °C/min (under nitrogen), from room temperature to 800 °C.
Preparation of the ligand sodium N-ethyl-N-Phenyldithiocarbamate, NaL
Carbon disulphide (6.0 mL, 0.1 mol) was added in small portions to an equimolar
mixture of sodium hydroxide dissolved in minimum quantity of water (4.0 g,
0.1 mol) and N-ethyl aniline (12.8 mL, 0.1 mol), and cooled in an ice bath at 0 °C.
After stirring for 3 h, the solidified mass was isolated from the solution and
recrystallized in acetone. The product was collected, rinsed with diethyl ether and
dried under vacuum over CaCl2 at room temperature [27].
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D. C. Onwudiwe, A. C. Ekennia
Preparation of the complexes ML2 (M=Co, Cu, Mn) and CrL3
Ethanol solution (20 mL) of the respective hydrated metal nitrate (Co(NO3)2Á6H2O:
0.87 g, 0.003 mol; Cu(NO3)2Á3H2O: 0.72 g, 0.003 mol; Mn(NO3)2Á4H2O: 0.75 g,
0.003 mol; Cr(NO3)3Á9H2O: 0.80 g, 0.002 mol) was added with stirring to 25 mL of
ethanol solution of sodium N-ethyl-N-phenyldithiocarbamate (1.31 g, 0.006 mol).
The reaction mixture was stirred for 1 h, and the colored, precipitated complexes
were filtered off, washed with water, then cold ethanol, followed by diethyl ether
and dried in vacuum.
[CoL2] (1), yield: 1.10 g, 81 %. Selected IR, m(cm-1): 1465 (C=N), 1278 (C2–N),
912 (C=S). Anal. Calc. for C18H20N2S4Co (451.56): C, 47.88; H, 4.46; N, 6.20; S,
28.40. Found: C, 47.22; H, 4.32; N, 6.10; S, 28.07 %. Electronic spectra (cm-1):
25,560, 37,857, 19,231, and 22,293 (e = 260 i mol-1 cm-1); Conductance
(X-1 cm2 mol-1):32.62; leff (B.M): 4.38.
[CuL2] (2), yield: 1.21 g, 88 %. Selected IR, m(cm-1): 1490 (C=N), 1282 (C2–N),
908 (C=S). Anal. Calc. for C18H20N2S4Cu (456.17): C, 47.39; H, 4.42; N, 6.14; S,
28.12. Found: C, 47.18; H, 4.42; N, 6.21; S, 28.47 %. Electronic spectra (cm-1):
26,750, 37,870, 19,567, and 22,390 (e = 203 i mol-1 cm-1); Conductance
(X-1 cm2 mol-1):18.32; leff (B.M): 1.95.
[MnL2]Á0.25H2O (3), yield: 0.94, 70 %. Selected IR, m(cm-1): 1460 (C=N), 1268
(C2–N), 920 (C=S). Anal. Calc. for C18H21N2S4O0.25Mn (452.07): C, 47.82; H,
4.57; N, 6.19; S, 28.37. Found: C, 48.22; H, 4.52; N, 6.20; S, 28.67 %. Electronic
spectra (cm-1): 25,340, 37,852, 09,231, and 12,293 (e = 324 i mol-1 cm-1);
Conductance (X-1 cm2 mol-1):48.09; leff (B.M): 5.72.
[CrL3]Á2H2O (4), yield: 0.88 g, 69 %. Selected IR, m(cm-1): 1472 (C=N), 1228
(C2–N), 912 (C=S). Anal. Calc. for C27H34N3S6O2Cr (676.96): C, 47.90; H, 5.06; N,
6.21; S, 28.42. Found: C, 48.02; H, 4.75; N, 6.43; S, 28.87 %. Electronic spectra
(cm-1): 25,176, 37,849, 23,729, and 17,280 (e = 65 i mol-1 cm-1); Conductance
(X-1 cm2 mol-1):26.32; leff (B.M): 3.20.
Preparation of [PdL2]Á4H2O (5)
Sodium N-ethyl-N-phenyldithiocarbamate (1.10 g, 0.005 mol) dissolved in 25 mL of
distilled water was added dropwise to 25 mL of cold aqueous solution of palladium
chloride (0.45 g, 0.0025 mol) in an ice bath at 0 °C. The reaction mixture was stirred
for 12 h, and the solids were filtered by suction, washed with water, ethanol and
diethyl ether, and dried invacuo. Yield: 0.92, 61 %. Selected IR, m(cm-1): 1497
(C=N), 1308 (C2–N), 920 (C=S). Anal. Calc. for C18H28N2S4O4Pd (571.10): C, 37.85;
H, 4.94; N, 4.91; S, 22.46. Found: C, 37.170; H, 4.62; N, 5.01; S, 22.37 %. Electronic
spectra (cm-1): 27,490, 38,650, 19,74, 22,398 (e = 310 i mol-1 cm-1); Conduc-
tance (X-1 cm2 mol-1): 28.07; leff (B.M): 0.34.
Antimicrobial studies
Antibacterial and antifungal screenings were conducted using a disc diffusion
method [28]. Petri plates were prepared with 20 mL of sterile Mueller–Hinton agar
123

Synthesis, characterization, thermal, antimicrobial and…
(MHA). The test cultures were swabbed on the top of the solidified media and
allowed to dry for 15 min. A specific amount (25 lL from the 100 lg/mL) of the
compound was introduced into each disc. The loaded discs were placed on the
surface of the medium and left for 30 min at room temperature for compound
diffusion. The negative control was dimethyl sulfoxide (DMSO). The plates were
incubated for 24 h at 37 °C for the bacteria and 48 h at 30 °C for the fungi strains.
Zones of inhibition were recorded in millimeters and the experiments were repeated
twice. The bacteria strains were E. coli, P. aureginosa, S. typhi and S. aureus, while
the fungi organisms were A. flavus and F. oxysporium. Streptomycin and
ketoconazole were used as the positive control drugs.
Minimum inhibitory concentration (MIC)
MIC studies of the complexes were carried out following a standard method for
bacteria and fungi [20]. Different concentrations (50, 40, 30, 25, 15, 10 lg/mL) of
the compounds were dissolved in DMSO (2 %). The solutions were added to each
medium in 96-well plates. An inoculum of 100 lL from each well was inoculated.
Ketoconazole (as an antifungal agent) and streptomycin (as antibacterial agent)
were included in the assays as positive controls. The plates for the fungi were
incubated for 48–72 h at 30 °C and 24 h at 37 °C for the bacteria. The MIC for the
fungi is the lowest sample concentration, showing no visible fungal growth after
incubation time. About 5 lL of the tested broth was placed on the sterile MHA
plates for bacteria and incubated at respective temperature. The MIC for bacteria
was determined as the lowest concentration of the compound inhibiting the visual
growth of the test cultures on the agar plate.
Antioxidant studies
To evaluate the antioxidant activity, stock solution (1 mg/mL) of the samples was
diluted to 100–500 lg/mL. Ethanol solution of DPPH (1 mL, 0.3 mmol) was added
to DMSO (2 mL) solutions of the sample at different concentrations [29]. The
mixture was shaken vigorously and allowed to stand at room temperature for
30 min. The absorbance was then measured at 517 nm using a UV–Vis
spectrophotometer. The lower absorbance of the reaction mixture indicates higher
free radical scavenging activity. Ethanol was used as the solvent and ascorbic acid
as the standard. The DPPH radical scavenger was calculated using the Eq. (1):
A1
DPPH Radical Scavenging Effect ð%Þ ¼ Ao À Â 100
ð1Þ
A0
A0 is the absorbance of the control reaction and A1 is the absorbance in the presence
of the samples or standards.
123

D. C. Onwudiwe, A. C. Ekennia
Results and discussion
Syntheses
The reaction between the ligand, sodium N-ethyl-N-phenyldithiocarbamate (NaL)
and Co(II), Cu(II), Mn(II), Cr(III) and Pd(II) salts resulted into the respective metal
dithiocarbamate complexes through a displacement reaction as shown in Scheme 1.
The coordination of the ligand to the respective metal ion in the complexes is by a
bidentate fashion through two sulphur atoms in a 2:1 stoichiometry for complexes
(1), (2), (3) and (5), but in a 3:1 stoichiometric reaction for complex (4). The metal
complexes are soluble in chloroform, dicholoromethane, acetone and coordinating
solvents like dimethylformamide (DMF) and dimethylsulphoxide (DMSO). The
analytical data are in good agreement with the proposed structure of the complexes.
Results of spectral and thermal studies of the complexes are consistent with their
M = Co, X = 6, n = 0; Cu, X = 3; n = 0; Mn, X = 4, n = 0.25
Scheme 1 Synthesis pathway for (I) Co, Cu, and Mn; (II) Pd and (III) Cr complexes
123

Synthesis, characterization, thermal, antimicrobial and…
formulation, and are in concordance with other similar dithiocarbamate complexes
[17–23].
FTIR spectral studies
The IR spectra of the complexes were compared with those of free ligand [27, 30] in
order to determine the coordination sites that may be involved in the complex
formation. All the complexes showed C–N stretching vibration of the dithiocar-
bamate moiety in the region of 1460–1497 cm-1. The position of this band is
indicative of the degree of the partial double-bond character in the C–N bond
(v(C=N) = 1640–1690; v(C–N) = 1250–1360 cm-1) [31–33]. In the spectrum of
the ligand, the C–N band was observed at 1453 cm-1 [27]. The appearance of this
band at a lower frequency in the free ligand compared to the metal complexes is
ascribed to the delocalization of electron toward the metal centre upon coordination
to the metal atoms [32–35]. The spectra of the Mn, Cr and Pd complexes (3, 4 and 5)
showed broad bands around 3430 cm-1 and sharp peaks around 1605 cm-1 which
could be ascribed to v(O–H) stretching and d(O–H) bending vibrations, respectively.
On the other hand, a decrease in the m(C–S) stretching band is observed relative to
the free ligand. The v(C=S) band is observed as an isolated band in the
908–920 cm-1 range in all the complexes and is indicative of a bidentate
dithiocarbamate [36].
Magnetic measurements and molar conductivity measurements
The magnetic susceptibility of all the synthesized complexes was measured in a
solid state at room temperature. The occurrence of high spin (d7) cobalt (II) was
confirmed by the observed magnetic moment value of 4.38 BM for (1). The
magnetic moment of high spin cobalt compounds in a cubic environment ranges
from 4.20–5.20 B.M and high spin octahedral Co (II) complexes are between 4.70
and 5.20 B.M, while tetrahedral Co(II) complexes have moments in the range
4.20–4.60 B.M [37]. Therefore, complex (1) was assigned a tetrahedral geometry. A
high spin d9 copper center was confirmed for complex (2) with a magnetic moment
of 1.95 B.M. Mononuclear Cu(II) complexes have magnetic moments in the range
1.9–22 B.M [38], which is usually higher than the spin-only value of 1.73 B.M due
to orbital contribution and spin–orbit coupling [39]. Hence, complex (2) is a
mononuclear four-coordinate complex. Chromium in a ?3 oxidation state is a d3
system. The Cr(III) complex, (4), has a magnetic moment of 3.2 B.M, which
supports the high spin octahedral geometry [34]. The Pd complex, (5), showed a
magnetic moment value of 0.34 B.M which is expected for a square planar geometry
for the palladium (II) complex due to diamagnetism [40]. The effective magnetic
moments of a high spin mononuclear Mn(II) complexes are expected to be close to
6
the spin-only value of 5.90 B.M. Since the ground term is A1 , there is no orbital
contribution [41]. A magnetic moment value of 5.72 B.M. was observed for
complex (3).
The metal complexes were dissolved in DMSO and the molar conductivities of
10-3 -M solution at room temperature were measured [42]. The low molar
123

D. C. Onwudiwe, A. C. Ekennia
conductance values (18.32–48.09 X-1 cm2 mol-1) of the complexes support their
non-electrolytic nature [43].
Electronic spectra
The electronic absorption bands of the complexes are classified into two distinct
groups. Transitions due to the ligand appeared in the UV region while d–d
transitions appeared in the visible region. These transitions were assigned with
respect to the structures of the complexes. The UV–visible spectra of the complexes
were recorded in DMSO and two bands were exhibited in the UV region around
25,340–27,490 cm-1 and 37,849–38,650 cm-1 due to n–p* and p–p* transitions,
respectively, from the dithiocarbamate ligand [19]. Additional bands around 23,729
(e = 80) and 17,280 cm-1 (e = 65) were observed in the visible region of the
4
4
spectra for the Cr(III) complex (4), which were assigned to A2g(F) ? T1g(F),
4
4A2g(F) ? T1g(P) transitions, respectively. This was in agreement with other
reported similar octahedral Cr(III) complexes [44]. Octahedral d9 copper complexes
2
are expected to have a broad bands due to 2Eg ? T1g with a 2D ground term due to
Jahn–Teller distortion. However, a Jahn–Teller effect caused by asymmetrical
2
filling of the, eg set of orbitals, causes a splitting of Eg and T2g into B1g, A2g
2
2
2
2
2
and B2g, Eg, respectively, which is usually observed [45]. Hence, the transitions
may lie within one envelope or may resolve into two or three absorption band
components. For tetrahedral d9 copper complexes, the effects of a Jahn–Teller effect
is less pronounced as it is the T2 set of orbitals that are asymmetrically filled. A
broad band below 10,000 cm-1 is usually observed in this case. Square planar d9
copper complexes usually show two bands of high energy above 10,000 cm-1
.
Therefore, the non-appearance of bands below 10000 cm-1 in the copper complex
indicates the absence of a tetrahedral geometry [38, 44]. The copper complex
showed absorption bands in the visible region at 19,567 cm-1 (e = 102) and
2
2
2
2
22,390 cm-1 (e = 108) due to B2g ? B1g and B2g ? A2g transitions of a
square planar geometry. This observation is similar to other reported copper (II)
complexes of square planar geometry [17]. The Co(II) complex (1) displayed two
bands, one at 15,806 cm-1 (e = 122) and one at 19,698 cm-1 (e = 156) which
4
4
4
4
were assigned to A2 ? T1(F) and A2 ? T1(P) transitions, respectively, of a
tetrahedral geometry [43, 44, 46, 47]. This supports the proposed geometry inferred
from the magnetic moment. The spectrum of the Pd(II) complex (5) showed two
absorption bands at 19,231 (e = 118) and 22,293 cm-1 (e = 125), typical of square
1
1
1
1
planar geometry, which are assigned to A1g ? B1g and A1g ? E2g transitions
[48]. The spectrum of the Mn(II) complex (3) showed two absorption bands at
11,985 cm-1 (e = 0.238) and 12,293 cm-1 (e = 347), typical of tetrahedral
4
4
geometry, which were assigned to 6A1g ? E1g and 6A1g ? A1g transitions
[49, 50].
Thermal decomposition of the complexes
The temperature range of the main decomposition stage, peak temperatures, and
weight of final residues, observed in each step of TG/DTG/DSC curves for
123

Synthesis, characterization, thermal, antimicrobial and…
Table 1 Thermal analysis data of complexes
Compounds
[CoL2]
[CuL2]
[CrL3]Á2H2O [PdL2]Á4H2O [MnL2]Á0.25H2O
Temp. range of main decomp. 261–375 197–360 241–414
(oC)
211–395
230–351
DTG peak T (oC)
Melting point
DSC peak, T (oC)
Mass of residue (mg)
Found
350
205
350
301
176
303
347
256
351
262
198
264
292
192
294
1.16
1.12
2.45
2.66
3.87
3.40
3.85
3.58
2.67
2.49
Calc.
complexes 1–5 are given in Table 1. In all the complexes, there was no formation of
a thiocyanate intermediate. Metallic thiocyanates are common intermediates in the
thermal decomposition of dithiocarbamate complexes [51–53].
Thermal decomposition of [CoL2] (1)
According to the TG and DSC curves (Fig. 1), complex (1) undergoes three
endothermic processes. The first stage, which occurred in the 145–165 °C
temperature range, is not associated with any weight loss and could be attributed
to a crystalline phase change [54], followed by a melting process at 205 °C. The
proposed anhydrous formulation from the elemental analysis result is supported by
lack of any mass loss up to 275 °C, indicating the absence of lattice or coordination
water molecules [55]. Finally, the third stage of decomposition which occurred
100
80
60
40
20
25
20
15
12
10 10
5
8
0
0
-5
0
200
400
600
800
Exo Up
Universal V4.7A TA Instruments
Temperature (°C)
Fig. 1 TG/DTG and DSC curves of the compounds [CoL2] (1) obtained in nitrogen atmosphere (75 mL/
min), heating rate 10 °C/min
123

D. C. Onwudiwe, A. C. Ekennia
between 261 and 375 °C is associated with the breakdown of the dithiocarbamate
ligand molecule and subsequent formation of CoS (found/calcd. mass of residue:
1.16/1.12 mg).
Thermal decomposition of [CuL2] (2)
The thermal decomposition of complex (2) involves two endothermic peaks
(Fig. 2). The first endothermic peak at 176 °C is the melting temperature. After
melting, the complex undergoes a fast mass loss of the most of the organic ligand in
the temperature range 197–360 °C, and this was accompanied by an endothermic
effect with peak temperature at 303 °C. According to the DTG curve, the maximum
rate of decomposition occurred at 301 °C and the resultant residue confirmed CuS
(found/calcd. mass of residue: 2.45/2.66 mg). In similar studies involving the
thermal behavior of copper (II) dithiocarbamates in nitrogen and air environments,
mass loss considerations at the main decomposition stages in N2 environments
indicated conversion of the complex to sulphides [56]. However, under the air
changes which corresponded to the formation of sulphide, oxidation of sulphide to
sulphate and the decomposition of sulphate to oxide were suggested [57]. The
thermal decomposition of copper bis(diethyldithiocarbamate) yielded cuprous
sulphide (Cu2S) nanocrystals [58].
Thermal decomposition of [MnL2]Á0.25H2O (3)
The decomposition pattern of the complex showed a weak endothermic peak at
135 °C, which is ascribed to the loss of lattice water. This stage was followed by an
endothermic decomposition stage which occurred without any observed weight loss
120
100
80
60
40
20
0
25
20
15
10
5
4
2
0
-5
1000
0
200
400
600
800
Exo Up
Temperature (°C)
Universal V4.7A TA Instruments
Fig. 2 TG/DTG and DSC curves of the compounds [CuL2] (2) obtained in nitrogen atmosphere (75 mL/
min), heating rate 10 °C/min
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Synthesis, characterization, thermal, antimicrobial and…
in the TG (Fig. 3). This is ascribed to the melting of the complex, and it is followed
by a huge weight loss in the temperature range 230–351 °C accompanied by a
strong endothermic effect in the DSC curve. The temperature of maximum rate of
decomposition for this stage is 292 °C. At the end of the decomposition, residues of
manganese sulphide were left (found/calcd. mass of residue: 2.67/2.42 mg). The
decomposition of Mn(II) complex of cyclic dithiocarbamates under air presented a
mass gain, probably due to the oxidation of the metal centre and Mn2O3 and MnSO4
as decomposition products. However, MnS was obtained under a nitrogen
atmosphere [59]. Similarly, MnO and MnS have been obtained as the final residue
of the thermal decomposition of manganese (II) diethyldithiocarbamate studied in
air and nitrogen environments, respectively [60].
Thermal decomposition of [CrL3]Á2H2O (4)
Thermal analysis showed that the first thermal effect on the decomposition of the
Cr(III) complex began at slightly above room temperature (Fig. 4) and continued up
to 105 °C. A weight loss corresponding to the removal of two molecules of water of
crystallization (DTG maximum at 80 °C) was observed with an endothermic peak at
84 °C. The anhydrous compound then melted around 256 °C, and afterwards the
liquid compound decomposed in a single well-defined step from 241 to 414 °C,
comprising the breakdown of the organic ligand and the formation of Cr2S3
(found/calcd. mass of residue: 3.87/3.40 mg). The thermal decomposition of a series
of Cr(III) dithiocarbamates studied in the 20–600 °C temperature range showed that
the final product of the decomposition was chromium sulphide Cr2S3 or with
100
80
60
40
20
0
40
30
20
10
0
10
5
0
-5
-10
1000
0
200
400
600
800
Exo Up
Universal V4.7A TA Instruments
Temperature (°C)
Fig. 3 TG/DTG and DSC curves of the compounds [MnL2]Á0.25H2O (3) obtained in nitrogen
atmosphere (75 mL/min), heating rate 10 °C/min
123

D. C. Onwudiwe, A. C. Ekennia
incompletely combusted sulphur atoms [61]. In similar studies [62], the residue was
a mixture of Cr2S3 and deficient sulphur chromium sulphides.
Thermal decomposition of [PdL2]Á4H2O (5)
The thermal behavior of complex (5) is in accordance with the proposed hydrated
formulation (Fig. 5). The presence of lattice (crystallisation) water could be
correlated to the first mass loss which began around 90 °C with a peak at 136 °C.
About 13 % mass loss occurred which corresponds to 4 molecules of water. Thus,
the first endothermic mass loss corresponds to the release of lattice water molecules
and the second endothermic peak at 198 °C (which occurred without any noticeable
weight loss) is ascribed to the melting temperature of the complex. In the following
step (211–395 °C), decomposition of the ligand molecule occurred. The thermal
decomposition ends at about 410 °C leaving as residue PdS2 (found/calcd. overall
mass loss: 3.85/3.58 mg). Thermal studies of palladium (II) complexes with
dithiocarbamate derivatives carried out in synthetic air showed the formation of PdS
as an intermediate product which further undergoes sulphur oxidation leading to the
partial oxidation of the PdS to PdO. Upon further heating, the compound
decomposed to metallic palladium [63]. TG analysis of a series of pyridine
solvated and non-solvated palladium dithiocarbamate complexes studied under
nitrogen atmosphere showed decomposition steps which leaves a residue of PdS
[64]. Palladium sulphide has been reported to exist in different phases such as PdS,
PdS2, Pd2.8S, Pd3S, Pd4S, Pd2.2S and Pd2.5S. Thus, the product of thermal
100
80
10
8
5
0
6
60
4
40
-5
2
20
0
0
200
400
600
800
Exo Up
Universal V4.7A TA Instruments
Temperature (°C)
Fig. 4 TG/DTG and DSC curves of the compounds [CrL3]Á2H2O (4) obtained in nitrogen atmosphere
(75 mL/min), heating rate 10 °C/min
123

Synthesis, characterization, thermal, antimicrobial and…
decomposition of complexes is influenced by the metal, the substituent on the
ligand, the heating rate and also the thermal environment [65].
Antimicrobial studies
The basic principle in the design of any active antimicrobial compound is to inhibit
the growth of pathogens without any side effects on treatment. Antimicrobial
activity of the ligands, metal complexes, and standard drugs were screened against
Gram-positive and Gram-negative bacterial strains to evaluate their potentials as
antibiotics. The antimicrobial analysis of the complexes was conducted and the
results are presented in Table 2 and Fig. 6. The complexes showed moderate to
good antimicrobial activity at a concentration of 100 lg/mL against the bacteria
strains (E. coli, P. aureginosa, S. typhi and S. aureus) and fungi organisms (A. flavus
and F. oxysporium). Complex 2 displayed the best activity against S. aureus
compared to other complexes and streptomycin. Similarly, the activities of (1) and
(4) exceeded streptomycin against S. aureus. Complex (4) showed the best activity
against P. aureginosa compared to the other complexes and the control drug.
Complex (3) had the best activity against S. typhi and E. coli compared to the other
complexes and the control drug. In general, the complexes displayed better activity
as antibacterial agents than as antifungal agents when compared to streptomycin and
ketoconazole. However, complex (5) did not show any antimicrobial activity and,
hence, was excluded from Table 2. This might be as a result of its bulkiness which
results in reduction in the cell membrane permeability of the complex. The
antimicrobial results of the complexes are comparable to similar works reported in
120
100
80
10
8
5
0
6
4
60
2
-5
40
0
20
-2
0
200
400
600
800
Exo Up
Universal V4.7A TA Instruments
Temperature (°C)
Fig. 5 TG/DTG and DSC curves of the compounds [PdL2]Á4H2O (5) obtained in nitrogen atmosphere
(75 mL/min), heating rate 10 °C/min
123

D. C. Onwudiwe, A. C. Ekennia
123

Synthesis, characterization, thermal, antimicrobial and…
Aspergillus flavus
30
25
20
15
10
5
Fasiparium oxysporium
Staphylococcus aureus
Pseudomonas aureginosa
Escherichia coli
Salmonella typhi
0
Compounds
Fig. 6 Histogram representing antimicrobial data for the complexes
literature for dithiocarbamate compounds [17]. Overall, the complexes showed
better antimicrobial activity than the free ligand due to chelation (Fig. 7).
The minimum inhibitory concentrations of the complexes were conducted at
different concentrations (50, 40, 30, 25, 15, 10 lg/mL,) and are presented in
Table 3. From the data, complexes with good antimicrobial activity also showed
low MIC which reflects their potentials as microbial inhibitors.
Antioxidant studies
DPPH Radical scavenging ability
2,2-Diphenyl-1-picrylhydrazyl (DPPH) is a stable organic radical compound and its
oxidative assay is used extensively in the quantification of the radical-scavenger
capacity or hydrogen-donor ability of samples. Antioxidants that exhibit DPPH
scavenging activity are continuously receiving attention. Compounds with antiox-
idant properties could be expected to offer protection in rheumatoid arthritis and
inflammation and to lead to potentially effective drugs [66, 67]. DPPH is a
stable free radical, which can accept an electron or proton radical and, in turn, gets
converted into a stable diamagnetic molecule. DPPH has a strong absorption band at
517 nm due to the presence of an odd electron in the molecule [68]. In the present
study, the ability of the ligand NaL and Cu(II) complexes to act as proton radical
(Na?) donors for the transformation of DPPH into its reduced form was
investigated. The absorbance decreases due to change of color from purple to
yellow as the radical is scavenged by the compounds. The antioxidant properties or
radical scavenging ability of sodium N-ethyl-N-phenyldithiocarbamate and its
123

D. C. Onwudiwe, A. C. Ekennia
123

Synthesis, characterization, thermal, antimicrobial and…
Table 4 Antioxidant data for the ligand and its Cu(II) complex
Concentration (lg/mL)
NaL
[CuL2]
Ascorbic acid*
100
200
300
400
500
18 0.512
23 0.704
34 0.105
39 0.110
42 1.535
48 0.610
51 0.610
55 2.194
60 0.747
75 1.220
57 0.431
62 0.863
63 0.863
67 0.932
87 1.220
No of replicates = 3, X SEM
* Standard
100
90
80
70
60
50
40
30
20
10
0
NaL
[CuL2]
*Ascorbic acid
0
100
200
300
400
500
600
Concentration ( g/mL)
Fig. 7 Trends in the inhibition of the DPPH radical by the ligand and its Cu(II) complex with standard
ascorbic acid at various concentrations
copper (II) complex is presented in Table 4 and compared to ascorbic acid (Fig. 7)
which is a standard antioxidant drug. The DPPH scavenging activity of Cu(II)
complex is significantly higher than that of the free ligand, indicating that this
complex is a much better/stronger free radical scavenger and antioxidant than the
ligand but lower when compared to ascorbic acid as a standard. Increase in the
concentration of the compounds in the antioxidant studies resulted in a
corresponding increase in the radical scavenging ability of the compounds.
Conclusion
Dithiocarbamate complexes of Co(II), Cu(II), Mn(II), Pd(II) and Cr(III) were
successfully prepared and characterized using different techniques. Thermal studies
showed a decomposition of the complexes into their respective metal sulphides, a
good characteristic for precursor compounds used for metal chalcogenide
123

D. C. Onwudiwe, A. C. Ekennia
nanoparticles and thin film synthesis. The ligand and metal complexes have been
screened to determine their biological properties, including the antifungal and
antibacterial activities under in vitro experimental conditions. The complexes
showed moderate to good antimicrobial activity and, overall, a better antimicrobial
activity than the free ligand due to chelation. The antioxidant activity of the ligand
and its copper complex was studied by DPPH free radical scavenging methods. The
ligand and the complex exhibited scavenging activity against DPPH free radicals
but the complex showed a significantly higher property than that of the free ligand,
indicating that this complex is a much better/stronger free radical scavenger and
antioxidant agent. This indicates that it can be a promising active component in the
field of biomedical application.
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