Synthesis, spectroscopic characterization, DFT calculations and biological evaluation of benzothiazole derivative bearing Mn(II) and Ni(II) metal ions

Article abstract of DOI:10.1016/j.molstruc.2017.06.051

N-(benzo[d]thiazol-2-yl)-3-oxo-3-phenylpropanamide ligand and its Nickel and Manganese complexes have been synthesized and characterized by elemental and thermal analyses, IR, diffuse reflectance, mass and UV–Vis spectra, molar conductance and magnetic moment measurements. The decomposition mechanism and thermal stability of the investigated complexes are interpreted in terms of their structures. The thermal behaviour of the complexes has been studied and different thermodynamic parameters are calculated using Coats–Redfern method. N-(benzo[d]thiazol-2-yl)-3-oxo-3-phenylpropanamide is a neutral bidentate ligand coordinating metal ions via thiazole ring nitrogen and amide carbonyl O forming high spin octahedral complexes with Mn(II) (2) and distorted square planar in case of Ni(II) (1). Natural bond orbital analysis and geometry optimization were carried out at DFT/B3LYP/6-31G(d) level of theory for the ligand and the mentioned complexes. Ab inito computations at the HF/6-31G(d) level of the theory is conducted in order to detect any probability of a hydrogen bond formation in the ligand. The dipole moment of the Ni(II) and Mn(II) complexes is recorded to be 9.69 and 7.39 Debye, respectively, indicating that the complexes are more polarized than the ligand 2.39 Debye. The in vitro biological activity of the metal chelates is screened against the Gram-positive bacteria (Staphylococcus aureus) and Gram-negative bacteria (Escherichia coli), fungus (Aspergillus flavus, Candida albicans). Ni(II) complexes displayed the highest activity against Candida albicans and Staphylococcus aureus with MIC values of 13, 30 μg/cm³, respectively.

Full text of DOI:10.1016/j.molstruc.2017.06.051

Journal of Molecular Structure 1147 (2017) 167e176
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Synthesis, spectroscopic characterization, DFT calculations and
biological evaluation of benzothiazole derivative bearing Mn(II) and
Ni(II) metal ions
Nadia E.A. El-Gamel , Korany A. Ali b
a, *
a
Chemistry Department, Faculty of Science, Cairo University, Gamma Street, 12613, Giza, Egypt
Applied Organic Chemistry Department, Center of Excellence and Advanced Materials, National Research Center, Dokki, 12622, Giza, Egypt
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 April 2017
Received in revised form
N-(benzo[d]thiazol-2-yl)-3-oxo-3-phenylpropanamide ligand and its Nickel and Manganese complexes
have been synthesized and characterized by elemental and thermal analyses, IR, diffuse reflectance, mass
and UVeVis spectra, molar conductance and magnetic moment measurements. The decomposition
mechanism and thermal stability of the investigated complexes are interpreted in terms of their struc-
tures. The thermal behaviour of the complexes has been studied and different thermodynamic param-
eters are calculated using CoatseRedfern method. N-(benzo[d]thiazol-2-yl)-3-oxo-3-phenylpropanamide
is a neutral bidentate ligand coordinating metal ions via thiazole ring nitrogen and amide carbonyl O
forming high spin octahedral complexes with Mn(II) (2) and distorted square planar in case of Ni(II) (1).
Natural bond orbital analysis and geometry optimization were carried out at DFT/B3LYP/6-31G(d) level of
theory for the ligand and the mentioned complexes. Ab inito computations at the HF/6-31G(d) level of
the theory is conducted in order to detect any probability of a hydrogen bond formation in the ligand.
The dipole moment of the Ni(II) and Mn(II) complexes is recorded to be 9.69 and 7.39 Debye, respectively,
indicating that the complexes are more polarized than the ligand 2.39 Debye. The in vitro biological
activity of the metal chelates is screened against the Gram-positive bacteria (Staphylococcus aureus) and
Gram-negative bacteria (Escherichia coli), fungus (Aspergillus flavus, Candida albicans). Ni(II) complexes
9
June 2017
Accepted 12 June 2017
Available online 19 June 2017
Keywords:
Benzothiazole
Metal complexes
NBO
Thermal analysis
Antimicrobial activity
displayed the highest activity against Candida albicans and Staphylococcus aureus with MIC values of 13,
3
30
m
g/cm , respectively.
©
2017 Elsevier B.V. All rights reserved.
1. Introduction
interactions.
Aromatic thiazoles, especially benzothiazoles, were reported to
have beneficial effects on animal metabolism and on various nat-
ural or marine compounds [2e4]. Benzothiazole derivatives are
known to possess interesting biological properties that show
antimicrobial, antitumor, anticonvulsant, anti-allergic, anti-in-
flammatory and fungicidal activities [5e9].
According to these benefits, it is motivating to study different
types of transition metal complexes of biologically active benzo-
thiazole ligand. In this work we report the synthesis, character-
ization and thermal behaviour of N-(benzo[d]thiazol-2-yl)-3-oxo-
3-phenylpropanamide (Fig. 1) and its complexes with Ni(II) and
Mn(II) cations, as well as the results of the investigation of their
antimicrobial properties to continue our research towards syn-
thesis of new compounds with antimicrobial activity and to un-
derstand modulation of the antimicrobial behaviour of the
chelating benzothiazole ligand. Geometrical optimization,
Antibiotics have been widely used against various types of dis-
eases, however nowadays there is a high apprehension concerning
the antibiotics loss of efficiency, caused by their abuse and misuse.
Therefore” the search for compounds with potential antimicrobial
activity caused by the worldwide development and spread of mi-
crobial resistance to various antibiotics [1] is of high priority.
However, the discovery of new drugs with new chemical structures
is time consuming and comes with quite highly cost. Thus, mo-
lecular structure modifications are considered the best method to
obtain desirable compounds by changing the physicochemical
properties according to the analysis of the drug-acceptor
“
*
Corresponding author.
E-mail address: nadinealy@hotmail.com (N.E.A. El-Gamel).
http://dx.doi.org/10.1016/j.molstruc.2017.06.051
022-2860/© 2017 Elsevier B.V. All rights reserved.
0

168
N.E.A. El-Gamel, K.A. Ali / Journal of Molecular Structure 1147 (2017) 167e176
calculation of the electronic configuration and electronic arrange-
ment besides nature of bonding were carried out at DFT/B3LYP/6-
2.1. N-(benzo[d]thiazol-2-yl)-3-oxo-3-phenylpropanamide
The synthesis of N-(benzo[d]thiazol-2-yl)-3-oxo-3-
31G(d) level of theory. We investigated their structural properties
by elemental and thermal analyses, magnetic moment, molar
conductance, IR, UVeVis and solid reflectance spectral techniques.
The natural bond orbital (NBO) character is used to identify and
phenylpropanamide has been reported [13e15]. Ethyl benzoyla-
cetate (2.5 g,13.31 mmol) was heated on oil bath at 150 C then (2 g,
ꢁ
13.31 mmol) of benzo[d]thiazol-2-amine was added gradually with
ꢁ
determine the effect of the resonating delocalized
locating in various sites within the molecule. This delocalization
could be accounted for * transition over the molecular skeleton.
p
-electrons
stirring. The reaction mixture was kept at 150 C for 30 min then
left to cool to room temperature. The reaction mixture was treated
with cold solution of ethanol then the precipitated product was
filtered off, washed with EtOH, dried and finally recrystallized from
ethanol to give of white crystals of N-(benzo[d]thiazol-2-yl)-3-oxo-
p-p
The synthesized complexes were screened for their antimicrobial
activity and it was found that, Ni(II) complex (1) had the highest
activity against Candida albicans (fungus) and Staphylococcus aureus
ꢁ
3-phenylpropanamide (3.6 g, 93%), m.p. 219 C.
þ
(Gram-positive bacteria). The higher biological activity compared
MS m/z (%): 296 [M ] (45). Anal. Calcd. For C16
12
H N
2
O
2
S
with Mn(II) complex (2) could be due to the different properties of
the metal ions upon chelation.
The high polarizability of the metal ions in the complexes plays a
vital role in the metabolic activity, by implying a labile “electron
cloud,” which can be attracted towards and shared with a ligand
(296.34): C, 64.85; H, 4.08; N, 9.45; S,10.82, found: C, 64.65; H, 3.88;
ꢀ1
N, 9.21; S, 10.62. FT-IR (cm ), imine
n
(C]N) 1578,
n(NeH) 3435,
*
n
(C]O) carbonyl 1638, (C]O) amide 1600.
n
lmax (nm): 276 p-p ,
*
342 n-
p .
[10e12].
2.2. Synthesis of the Ni(II) and Mn(II) complexes
Density functional theory (DFT) calculations showed that the
ꢁ
dipole moment of Ni complex (1) (9.69 Debye) is higher than the
dipole moment of the Mn complex (2) (7.39 Debye) “therefore” the
activity of the Ni complex (1) is enhanced due to its higher polarity;
this consequently increases its hydrophilic character, favoring its
penetration through the bacterial/fungal membrane, causing
changes in cell permeability characteristics leading to cell damage
and finally its death.
A hot water-ethanolic solution (60 C) of the metal chloride salts
for Ni(II) and Mn(II) (25 mL, 10 mmol) was added to a hot ethanolic
solution of N-(benzo[d]thiazol-2-yl)-3-oxo-3-phenylpropanamide
(10 mmol) in ethanol (10 mL. The reaction mixture was refluxed
for 2 h, where the complexes precipitated as fine powders. The
solid complexes were filtered, washed with ethanol, then with
diethyl ether, dried in a vacuum desiccator over anhydrous CaCl
2
and obtained as a solid powder, yield 75e90%. Several trials have
been done in order to get suitable crystals for X-ray single mea-
surement; “however” the obtained crystals were too small and not
2
. Experimental
All chemicals used were of highest purity available and analyt-
suited for X-ray single crystal technique as shown in Fig. 2.
þ
ical reagent grade (AR): dioxane (Aldrich), Ethyl benzoylacetate
Sigma), methanol (Aldrich), MnCl (Sigma), NiCl $6H O, (BDH),
absolute ethyl alcohol, diethyl ether (Acros), Yeast extract and agar
Sigma).
FTIR spectra were obtained from dispersions in KBr using a
PerkineElmer FT-IR type 1650 spectrophotometer. The spectra
[Ni(L)Cl
427, 356, 150, 135, 120, 78. Anal. calcd. for C16
41.59; H, 3.49; N, 6.06; found: C, 41.62; H, 3.43; N, 6.21%. FT-IR
2
]$2H
2
O (1): Yield 90%; dark green MS m/z: 463 [M þ1],
(
2
2
2
16 2 4 2
H N O SNiCl : C,
ꢀ
1
(
(cm ), imine
n
(C]N) 1612,
n(C]O) carbonyl 1638, n(C]O)
amide 1597,
n(MeO) stretching vibrations of amide carbonyl 518,
n
(MeN) stretching vibrations of thiazole ring nitrogen 556. l
max
ꢀ
1
*
*
*
were collected in the range from 400 to 4000 cm with a reso-
(nm): 276
p-p
, 345 n-
O)
] (2): Yield 75%; brown MS m/z: 459 [M þ1],
422, 351,151,121, 78. Anal. calcd. for C16 SMnCl : C, 41.94; H,
3.52; N, 6.11; found: C, 41.97; H, 3.51; N, 6.14%. FT-IR (cm ), imine
(C]N) 1591, (C]O) carbonyl 1638, (C]O) amide 1592, (MeO)
stretching vibrations of amide carbonyl 510, (MeN) stretching
vibrations of thiazole ring nitrogen 550. max (nm): 276
p , 450 n-p .
ꢀ1
þ
lution of 2 cm . The mass spectra were recorded by the EI tech-
nique at 70 eV using MS-5988 GS-MS HewlettePackard
instrument. UVeVis spectrophotometric measurements were car-
ried out using automated spectrophotometer UVeVis Thermo
Fischer Scientific Model (Evolution 60) ranged from 200 to 900 nm.
Molar conductivity was measured on a ELICO(CM82T) conductivity
bridge. The molar magnetic susceptibility was carried out on
powdered samples using the Faraday method. The diamagnetic
[Mn(L)Cl
2
(H
2
2
H
16
N
2
O
4
2
ꢀ1
n
n
n
n
n
*
l
p-p , 346 n-
*
*
p
, 443 n-
p , 456 MLCT.
4
corrections were made by Pascal's constant and Hg[Co(SCN) ] was
2.3. Theoretical calculation
used as a calibrant. The solid reflectance spectra were performed on
a Shimadzu 3101pc spectrophotometer. Thermal analyses were
carried out using a Shimadzu TGA-50H and DTA-50H thermogra-
vimetric analyzer in a dynamic nitrogen atmosphere (flow rate
Ab inito computational algorithms, involving the natural bond
orbital analysis (NBO) [16], density functional theory (DFT) calcu-
lations were carried out by the Gaussian 09W [17]. The dipole
moments, and geometric parameters were computed by applying
the B3LYP method with a medium basis set. All structures were
optimized at the restricted DFT/B3LYP/6-31G(d) level of theory
with the natural population analysis. The change in the natural
hybrid orbital which directing to the NBO character of the two-
centered bond was explored to recognize and determine the ef-
ꢀ ꢀ1
1
ꢁ
2
0 mL min ) with a heating rate of 10 C min . The percentage
ꢁ
weight loss was measured from ambient temperature to 600 C,
highly sintered a-Al was used as reference.
2 3
O
fect of the resonating delocalized
molecule.
p-electrons located within the
2.4. Antimicrobial activity
The antimicrobial activity of the tested samples was determined
using a modified KirbyeBauer disc diffusion method [18,19], and
their efficacy was assessed by using the Minimum Inhibitory
Fig. 1. N-(benzo[d]thiazol-2-yl)-3-oxo-3-phenylpropanamide.

N.E.A. El-Gamel, K.A. Ali / Journal of Molecular Structure 1147 (2017) 167e176
169
Fig. 2. The obtained crystals of (a) Ni(II) and (b) Mn(II) complexes under microscope (the length of the scale is 500 mm).
Concentrations (MIC) method [20]. In the disc diffusion method,
and N14eH15/N12 (2.42452 Å); respectively (Scheme 1). The
3
3
1
00 mm of the test bacterium/fungus was grown in 10 cm fresh
bands arising from
are located in the literature at 1680 and 1630 cm ; respectively
[23], these bands were observed at 1638, 1600 cm in the exper-
n
(C]O) vibrations of carbonyl and amide groups
3
ꢀ1
medium till having a count of roughly. 108 cells/cm for bacteria or
3
3
ꢀ1
105 cells/cm for fungi [21]. Microbial suspension (100 mm ,
3
2
0 mg/cm ) was spread onto agar plates. Plates inoculated with
imental spectrum of ligand, a significant shift to lower wave-
numbers is observed due to the presence of available hydrogen
bonds association between amide carbonyl oxygen and its neigh-
filamentous bacteria Gram-positive bacteria (Staphylococcus aureus)
ꢁ
were incubated at 35e37 C for 24e48 h, and for fungus (Candida
ꢁ
albicans) at 30 C for 24e48 h, and the diameters of the inhibition
boring
hydrogen
atoms
C16eO19/H32
(2.48476
Å),
zones were measured in mm [22]. Filter discs impregnated with
C16eO19/H27 (2.88356 Å), C16eO19/H33 (3.06917 Å),
C16eO19/H15 (3.12796 Å) and between carbonyl oxygen and its
neighboring hydrogen atoms C18eO20/H33 (2.46450 Å),
C18eO20/H25 (2.48306 Å), C18eO20/H15 (2.48797 Å). (Scheme
1).
3
10 mm solvent (distilled water, chloroform, DMSO) were used as
negative control, while amphotericin B (antifungal agent) served as
positive control. In the MIC method, standard culture media of
bacteria (Staphylococcus aureus) (ATCC 19433) and fungus stain
3
(
Candida albicans) (ATCC 10231) were vaccinated into 5 cm liquid
In the experimental IR spectra of the complexes, the carbonyl
band remained unchanged upon coordination confirming non-
participation in the chelation, while the amide group shifted to
soybean, casein, and digest (SCD) medium and cultured for 24 h at
ꢁ
3
5.5 C. Ampicillin trihydrate and clotrimazole were applied as
ꢀ1
standard antibacterial and antifungal agents; respectively. The
lower frequencies by 3e8 cm indicating significant involvement
of the amide group in the coordination.
cultured fluids were adjusted to reach 105e106 microorganisms
3
per cm , and used for vaccination in the MIC test.
The literature value of the imine C]N stretching vibration of the
3
ꢀ1
Fungus strain was vaccinated into 5 cm glucose polypeptone
thiazole moiety is located at 1580 cm [23], but in our case the
ꢁ
ꢀ1
(
GP) medium and cultured for 48 h at 30 C. The cultured fluids
value is 1578 cm , as observed in an experimental spectrum of the
3
were adjusted to concentration of 106e107 cm , and used for
ligand, a slightly shift to lower wavenumbers maybe attributed to
the effect of the presence of soft hydrogen bonds association be-
tween N14eH15/O20 (2.48797 Å) and N14eH15/N12
(2.42452 Å). The thiazole ring allows coordination of the metal ions
through donation of electrons offered by sulfur or nitrogen. How-
ever, coordination through thiazole nitrogen is more common
rather than the sulfur besides nitrogen considered as a better donor
atom than sulfur, additionally, hard acids [Mn(II) and Ni(II)] coor-
dinate to hard base through nitrogen atom of the thiazole ring.
Furthermore, the charge density on nitrogen atom (N12) of the
vaccination in the MIC test. The Ni complex was suspended in the
3
solvent, and several dilutions (0e320
m
g/cm ) were made and
3
mixed with 5.0 cm of the standard organisms, afterwards added to
ꢁ
the plates and incubated for 24 h at 35 C.
The microorganism suspensions at 106 colony-forming units
3
(
CFU)/cm concentration were vaccinated into the corresponding
wells. The growth of the microorganisms was checked for this
period. When no growth of microorganism was detected in the
medium containing the lowest concentration of test material, the
MIC of the test material was determined at this step of dilution.
ꢀ
3
thiazole ring of the ligand was found to be ꢀ0.523 cm while
sulfur atom (S13) in thiazole ring of the ligand recorded positively
ꢀ
3
charge 0.268 cm as shown in Scheme 2. This initiates the suit-
ability of the nitrogen atom over sulfur atom in the thiazole ring for
coordination to the positively charged metal ions.
3
. Results and discussion
3.1. Vibration spectra
In the prepared complexes, the experimental value of the C]N
ꢀ1
stretching vibration band is found at 1612 and 1591 cm for Ni(II)
and Mn(II) complexes; respectively confirming coordination
through the heterocyclic nitrogen of the thiazole [24]. This may be
A detailed interpretation of IR bands of the ligand and the metal
ꢀ
1
complexes is discussed. The presence of broad band at 3435 cm
assigned to
n(NeH) stretching band. The literature value of n(NeH)
ꢀ1
explained in terms of the highly mobile p-electrons within the
stretching band appears at 3400 cm [23] but the experimental
ꢀ1
thiazole conjugated systems upon its coordination through the
endocyclic N, where the coordination of the ligand takes place
value is 3435 cm , Upon forming hydrogen bonds, NeH stretching
band in the infrared spectrum tend to shifts to lower frequency, due
to the probability of multiple intramolecular hydrogen bonds
formed between NeH and/or terminal C]O and thiazole ring ni-
trogen resulting in hydrogen bonds N14eH15/O20 (2.48797 Å)
3
through their N atom in a monodentate mode. The experimental
ꢀ1
stretching vibration of
n
(CeS) of the ligand is located at 720 cm
;
this band remained unchanged upon chelation which further

170
N.E.A. El-Gamel, K.A. Ali / Journal of Molecular Structure 1147 (2017) 167e176
Scheme 1. Optimized structure of the ligand at the B3LYP/6-31G(d) level of theory.
supports the coordination via thiazole nitrogen.
hybrid orbitals [25]. In fact, the delocalized bond C7eN12 was
99.56% p-character in N 12 and 99.80% p character in C7, which
shows the resonating p-electrons.
ꢀ
1
New near eIR absorbance bands at 518e510 cm
and
ꢀ1
6
12e556 cm are shown in the experimental spectra of the metal
complexes refer to (MeO) and (MeN) stretching vibrations of
n
n
The two-centered bond between C16 and O19 shows more p
than s character as seen by the 66.96% p character on C16 and
54.46% p character on O19, as well as the two-centered bond be-
tween N14 and C16 shows more p than s character as seen by the
63.55% p character on N14 and 68.42% p character on C16. However,
N12 exhibits higher electronegativity obvious from its higher value
of charge density (ꢀ0.876) comparing to that of N14 (ꢀ0.832) as
shown in Scheme 2, therefore there is a transfer interaction
occurring down the chain of conjugated bonds heading towards the
benzothiazole moiety.
amide carbonyl and thiazole ring nitrogen; respectively [10,12].
Resonance of the double bond is observed in the ligand by
checking the second order Perturbation Theory Analysis in NBO
basis at the B3LYP/6-31G(d) level of theory, due to the delocaliza-
tion of electronic occupancies in the hybrids locating for the two
centered double bond formed between atoms C16eO19 and
C7eN12 in addition to in the benzene ring. “Thus” double and
single bond hybrid orbitals were seen from the delocalization of
2
these bonds. In this molecule for its interaction nitrogen used sp
Scheme 2. Total density of electron distributions of benzothiazole ligand.

N.E.A. El-Gamel, K.A. Ali / Journal of Molecular Structure 1147 (2017) 167e176
171
With regard to Scheme 2, the electron density is quite different
between the two benzene rings, this may due to the presence of
more electronegative atoms with high charge density O20
ꢀ
3
ꢀ3
ꢀ3
(
ꢀ0.486 cm ), O19(-0.496 cm ), N14(-0.679 cm ) near the
benzene ring which draw electron density away from the benzene
ring, rendering it more electron poor, whereas in case of the other
benzene ring in the benzothiazole, the electron density is quite
moderate by neighboring N12 with high electronegativity
ꢀ
3
ꢀ3
(
ꢀ0.523 cm ) and S13 with positive charge (0.268 cm ), there-
fore the electron density in case of the benzene ring near the
thiazole ring would not reduce too much.
It is worth to mentioned that the transfer of the lone pair of N14
to
electron stabilization energy E(2) of 83.76 kcal mol , in addition,
the transfer of the lone pair of N14 or of S13 to * orbital of C7eN12
is quite a stable interaction with a two-electron stabilization energy
p* orbital of C16eO19 is the most stable interaction with a two-
ꢀ1
p
ꢀ
1
E(2) of 57.18, 45.45 kcal mol ; respectively.
The C7eN12 bond will, in turn, donate its
p-electrons to the
benzene ring via the antibonding orbital of C1eC2, with an E(2) of
ꢀ1
2
5.22 kcal mol , however C1eC2 donates its electrons in turn, to
the * orbital of C7eN12 and being stabilized by an E(2) of
4.39 kcal mol . The stabilized energies are determined by
applying B3LYP method with a medium basis set. This band is
observed at 276 nm and is assigned to * transition as shown in
p
ꢀ
1
Fig. 3. Experimental UVeVis absorption spectra of the ligand and Ni(II) (1) and Mn(II)
1
(
2) complexes.
p-p
the experimental UVeVis absorption spectrum (Fig. 3).
Thus, it may be stated that the benzothiazole ligand under study
carries strong resonating delocalization along its skeleton and its
neighboring N14eC16.
The NBO supports this discussion as the N12 lone-pair is seen to
have an occupancy of 1.99928 electrons in its Lewis NBO whose
energy is ꢀ15.33603 a.u; while the N14 lone-pair is seen to have an
occupancy of 1.99923 electrons with an energy of ꢀ15.43708 a.u.
However, N14 appears to bifurcate its lone-pair delocalization
charges could be determined by distributing of the atomic charges.
The dipole moment of the Ni(II) and Mn(II) complexes is 9.69 and
7.39 Debye; respectively indicating that the complexes are more
polarized than the ligand (2.39 Debye) as recorded in Table 1 (see
ESI).
Molecular electrostatic potential (MEP) map is highly recom-
mended and used for predicting sites and relative reactivity to-
wards electrophilic and nucleophilic attack. In case of the ligand the
negative (red) regions are mostly centered over the nitrogen and
oxygen atoms of the ligand and hence it would be predicted that
these sites are the most reactive and suitable sites for electrophilic
attack as shown in Scheme 2.
by remotely interacting with the
C7eN12 bonds. This * orbital in which C16 (33.93%) shares with an
sp hybrid holding 33% s character and 67% p character, while O19
66.07%) offers its 45.03% s character and 54.46% p character.
Whereas, the
* orbital in which C7 (40.99%) shares with an sp2
hybrid holding 37% s character and 63% p character, however N12
59.01%) offers its 39% s character and 61% p character. The lone-
pair N12 of the C]N group shows n- * transition as it holds the
largest electron occupancy in the * orbital of the C7eN12 bond
and it displayed as a shoulder band at 342 nm in the UVeVis data
Fig. 3).
The atomic charges calculation considered as a potential
p* orbital of the C16eO19 and
p
2
(
p
3.2. Natural bond orbital analysis and molecular electrostatic
potential maps
(
p
p
NBO theory [16,26] has been used to give useful information
about the natural charge of the metal center and the electronic
arrangement. The electronic arrangement of Ni(II) in Ni complex is:
(
0
.38
8.39
0.49
0.01
[Ar]4s 3d 4p 5p
and distributed as 17.99605 core, 8.89
parameter to represent the electronic characteristics of molecular
system. The charge density on nitrogen atom (N12) of the thiazole
valence and 0.01462 Rydberg electrons presenting total 27.28067
electrons and remaining þ0.71933 e as a positive charge. The oc-
ꢀ
3
1.96854
1.89531
1.88275
ring of the ligand was found to be ꢀ0.523 cm ; Scheme 2. This
initiates the suitability of the nitrogen atom in the thiazole ring for
coordination to the positively charged metal ions. Whereas, the
positive charge on sulfur atom (S13) in thiazole ring of the ligand
cupancies of Ni 3d-orbitals are: dxy
d
xz
d
yz
d
x2-y2
1.93441
1
.59104 2 1.04951
d
z
. The occupancies of Cl (33) 3p-orbitals are: p
x
1
.84043
1.83403
p
y
p
z
; the occupancies of Cl(34) 3p-orbitals are: p
1.75272.
The results from NBO analysis show that the
x
1
.94987
1.90366
p
y
p
z
ꢀ
3
(
0.268 cm ) dismiss the possibility of sulfur as binding site. In
s
(NieCl33) and
addition, the charge density on oxygen atom of the carbonyl amide
s
(NieCl34) bond are strongly polarized towards the chlorine
ꢀ
3
ring (O19) of the ligand was found to be (ꢀ0.496 cm ); this value is
atoms, with about (78.99e80.44) % of electron density concen-
trated on the chlorine atoms; therefore the bond length of NieCl33
(2.1548 Å) is quite similar to the bond length of NieCl34 (2.1535 Å).
The optimized geometries of the complexes are given in Scheme 3.
Selected bond lengths and angles are given in Table 1 (see ESI).
The coordination sphere around the metal ion is formed from
neutral bidentate ligand; bonding via C]N in the thiazole ring and
amide carbonyl group C]O; the two coordinate NieO18 and
NieN12 bond lengths are 2.362 and 1.841 Å; respectively. The
average of NieN12 and NieO18 bond distance of 1.841 and 2.362 Å;
respectively is comparable to the NieN and NieO bond distance
found in similar compounds [27].
higher than the value found on the terminal carbonyl oxygen O20
ꢀ
3
(
ꢀ0.486 cm ). This favours the suitability of carbonyl amide oxy-
gen for coordination. It has been found that in the prepared com-
plexes, the charges on the coordinated nitrogen and oxygen atoms
ꢀ
3
ꢀ3
are increased to (ꢀ0.600 cm ) and (ꢀ0.502 cm ) in case of Ni
ꢀ3
ꢀ3
complex (1) and ꢀ0.643 cm
complex (2); respectively which indicates the back donation from
the metal sites in a MLCT mode to the * orbitals of the ligand
Scheme 3).
Furthermore, the direction of the dipole moment vector in the
complexes which based on the centers of negative and positive
and -0.576 cm
in case of Mn
p
(

172
N.E.A. El-Gamel, K.A. Ali / Journal of Molecular Structure 1147 (2017) 167e176
Scheme 3. Optimized molecular structure of the prepared complexes (a, Ni complex (1), b, Mn complex (2) with charge distribution, hydrogen atoms are omitted for simplicity.
0
.95664
0.82402
The electronic arrangement of Mn(II) in Mn complex is: [Ar]
The occupancies of O(33) 2p-orbitals are: p
x
p
y
p
z
S0 3d 4p 5p
.31
5.24
0.47
0.01
0.81567
0.84399
0.94502
4
and distributed as 17.99498 core, 6.03
; the occupancies of O(36) 2p-orbitals are: p
x
p
y
0
.80649
valence þ 0.01696 Rydberg electrons presenting total 24.04194
p
z
. Therefore the bond length of Mn41eO36 (2.09023 Å) is
shorter than Mn41eO33 (2.113 Å) due to the variance in the
contribution of P and d character.
electrons and remaining þ0.95806 e as a positive charge. The oc-
1
.61828
1.19058
1.29988
cupancies of Mn 3d-orbitals are: dxy
d
xz
d
yz
d
x2-
0
.48440 2 0.65003
y2
d
z
. The occupancies of Cl(39) 3p-orbitals are: p
x
The average of Mn41eN12 and Mn41eO18 bond distance of
1.964 and 1.935 Å; respectively is comparable to the MneN and
MneO bond distance found in similar compounds [28,29].
As we mentioned before that Molecular electrostatic potential
(MEP) map is considered as a useful tool for predicting sites and
relative reactivity towards electrophilic and nucleophilic attack. In
case of the complexes, different values of the electrostatic potential
are shown by different colors as shown in Fig. 4: most of positive
0
.97198
0.96852
0.90990
p
y
p
z
; the occupancies of Cl(40) 3p-orbitals are:
0
.96978
0.98218
0.91768
p
x
p
y
p
z
. Thus the bond length of Mn41eCl39
(
2.36579 Å) is shorter than Mn41eCl40 (2.44435 Å) due to the
inequality in the contribution of P and d character. Additionally, the
(MneCl39) and (MneCl40) bond are strongly polarized towards
s
s
the chlorine atoms, with about (74.02e73.58) % of electron density
concentrated on the chlorine atoms.

N.E.A. El-Gamel, K.A. Ali / Journal of Molecular Structure 1147 (2017) 167e176
173
electrostatic potential regions are shown by blue color, the regions
of the most electro-negative potential are shown by red color and
green shows regions of zero potential. Potential increases in the
following order: blue > green > yellow > orange > red.
complex is both spin and Laporte' forbidden [37].
Mn(II) (2) complex exhibited a weak bands at 35,714, 27,778 and
ꢀ
1
6
4
4
22,727 cm . These band may be assigned to
A
1g
/
T
1g( P);
/ A1g( G) transitions, respectively.
These bands suggest octahedral geometry of the Mn(II) complex
[38,39]. 5.89 B is recorded for Mn(II) complex (2) corresponds to
octahedral geometry [38,39].
6
4
4
6
4
4
4
1g g 1g g
A / E ( D); A / E
With regard to Fig. 4, Mn ion is surrounded by six centers of high
ꢀ
3
ꢀ3
electron density Cl (ꢀ0.631 cm ), Cl (ꢀ0.593 cm ),
O
O
m
ꢀ
3
ꢀ3
ꢀ3
(
ꢀ0.576 cm ),
N
(ꢀ0.643 cm ),
O
(ꢀ0.792 cm ),
ꢀ3
ꢀ1
(
ꢀ0.798 cm ) whereas Ni ion (a) is surrounded by four atoms of
A broad band centered at about 21,882 cm is observed in the
absorption spectrum (Fig. 3) maybe assigned to MLCT transition
from Mn to carbonyl as well as the C]N groups.
ꢀ
3
ꢀ3
lower electron density Cl (ꢀ0.417 cm ), Cl (ꢀ0.412 cm ), O
ꢀ
3
ꢀ3
(
ꢀ0.502 cm ), N (ꢀ0.600 cm ); therefore the electron density is
ꢀ
1
higher near Mn ion (b) than that near Ni ion (a).
Ni(II) complex shows two bands at 19,230 and 21,277 cm ,
1
1
1
1
The calculated energies of the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
which are assigned to A1g / A2g and A1g / B1g transitions,
respectively, suggesting the predominance of the square planar
(
LUMO) of the ligand and their Ni(II) (1) and Mn(II) (2) complexes
stereochemistry [40]. The
meff of 2.25 mB is distinctive of the sug-
are calculated. The energies of the HOMO and LUMO orbitals of the
ligand and their complexes were negative which elucidate the
stability of the ligand and the complexes [30]. The energy gap be-
tween the HOMO and LUMO is calculated as shown in Fig. 5. The
measured values of the energy separation between the HOMO and
LUMO for the reported complexes are consistent with the stable
transition metal complexes [31].
gested for distorted square planar geometry; where the NieCl
plane is not parallel with respect to NieNeO plane. The inter-
planar angle between both ions in two different planes is not zero.
2
3.4. Thermal analysis
Simultaneous thermogravimetric (TG) and differential thermal
(DTA) analyses were applied to study the thermal behaviour of the
3
D plots of the LUMO for Ni (II) (1) and Mn(II) (2) complexes are
ꢁ
shown in Fig. 5. Analysis of the frontier molecular orbitals (FMOs) of
complexes 1 and 2 showed that their HOMOs are mainly localized
on the benzothiazole moiety and chlorine with slight contribution
from phenyl ring and in LUMO more electronic density is localized
on metals center. The HOMO level is mostly localized on the ben-
zothiazole ring especially, O(19) N(12) and S(13) atoms, confirming
that these atoms are the preferred sites for nucleophilic attack.
complexes up to a temperature of 600 C as displayed in Fig. 6. The
thermal decomposition of the ligand occurs in one decomposition
ꢁ
step within the range 174e572 C, a complete decomposition is
observed with an estimated mass loss of 99.87% (calcd. 100%)
(Fig. 6a). DTA displays two successive endothermic signals at 225
ꢁ
ꢁ
and 390 C and one exothermic signal at 526 C due to the complete
disintegration of the ligand (Fig. 6b).
Thermal decomposition of (1) comprises five successive
ꢁ
3.3. Absorption spectra, molar conductivity and magnetic
decomposition steps within the temperature range 45e600 C, it
susceptibility measurements
starts with the liberation of the hydrated water molecules between
ꢁ
4
5 and 134 C with an estimated mass loss of 8.09% (calcd. 7.79%)
ꢁ
The molar conductivity measurements of the complexes in DMF
solution were recorded 13.50, 18.80
(Fig. 6c). DTA presents a small exothermic peak at 60 C (Fig. 6d).
ꢀ
1
2
ꢀ1
ꢁ
U
cm mol for (1), (2),
The second decomposition step at 135e188 C corresponds to
respectively. These relatively low values indicated non-electrolytic
kind of the complexes [32e36]. The solid electronic spectra of
Mn(II) complex are characterized by weak spin forbidden transi-
liberation of Cl
2
with a mass loss of 15.52% (calcd. 15.35%). This step
ꢁ
is connected with one endothermic effect at 163 C. The third,
fourth and fifth mass loss steps occur in the range of 189e600 C
due to the decomposition of the rest of organic ligand as
ꢁ
5
tion; this maybe due to the LF transitions in high-spin Mn(d )
Fig. 4. Molecular electrostatic potential map for (complexes (1, a) and (2, b). The electron density isosurface is 0.0004 a.u.

174
N.E.A. El-Gamel, K.A. Ali / Journal of Molecular Structure 1147 (2017) 167e176
Fig. 5. HOMO and LUMO orbitals and energy gap of ligand and complexes, red regions (negative) and green regions (positive). (For interpretation of the references to colour in this
figure legend, the reader is referred to the web version of this article.)
C
16
H
12
N
2
OS (83.81%, calcd. 83.83%) and formation of NiO as a res-
in vibrational as well as rotational and translational states or may
refer to alterations in mechanical potential energy of the prepared
complexes.
It also observed that all the complexes recorded negative values
of the entropy values, which confirm the spontaneous formation of
the activated complexes besides they have more favored and or-
dered systems than reactants [42e45].
idue (16.19%, calcd. 16.17%) (Fig. 6c). These steps are accompanied
by one endothermic peak at 221.5 C and three exothermic peaks at
ꢁ
ꢁ
3
28, 398 and 450.4 C (Fig. 6d).
According to the TG profile of (2), the decomposition occurs in
four well defined decomposition steps within the range of
ꢁ
5
0e600 C (Fig. 6e). Liberation of hydrated water may occur in the
first step connected with two endothermic process at 60 and
ꢁ
1
7
20.08 C (Fig. 6f), respectively, with a mass loss of 7.738% (calcd.
3.5. Antimicrobial studies
ꢁ
.863%). The second step, within the range 151e260 C, can be
assigned to evolution of chlorine gas with an estimated mass loss of
In order to estimate the antimicrobial activity of the prepared
complexes, variety of organisms have been used in order to in-
crease the chance of identify and monitor the biological efficiency
of the complexes under study. The studies are carried out on the
bacteria Escherichia coli (Gram-negative) Staphylococcus aureus
(Gram-positive), and the fungi Candida albicans and Aspergillus fla-
vus using the assay plates disc method on appropriate nutrient
medium. The inhibition zone diameters after incubation all the
prepared complexes are displayed in Fig. 7. On comparing the
biological activity of the ligand and its metal complexes with the
standard (tetracycline, antibacterial) and (Amphotericin B, anti-
fungal), we found that the ligand does not show any biological
activity, whereas Ni (1) and Mn (2) complexes showed biological
activity (Fig. 7). However, Ni complex exhibited a significant inhi-
bition towards the growth of Staphylococcus aureus (Gram-positive
bacteria) and Candida albicans.
1
5.342% (calcd. 15.47%). DTA presents two exothermic signals at
ꢁ
178.17 and 215.6 C. The last two steps could be attributed to the
decomposition of the remaining ligand as C16
H
12
N
2
S in the range of
ꢁ
2
61e600 C with an estimated loss of 56.938% (calcd. 57.257%).
These steps are connected with two exothermic effects at 321.5 and
ꢁ
4
88 C (Fig. 6f). MnO
2
remains as a final residue with a mass loss of
1
8.972% (calcd. 19.41%).
The kinetic parameters like activation energy (E*), enthalpy
H*), entropy ( S*) and free energy change of decomposition
G*) were estimated graphically by applying the CoatseRedfern
(D
D
(D
relation [41]. The thermodynamic parameter data are summarized
in Table 2 (see ESI). The activation energies of decomposition were
ꢀ
1
recorded to be 730.94 and 322.58 kJ mol for Ni and Mn com-
plexes; respectively. The high values of the activation energies
assert and confirm the thermal stability of the prepared metal
complexes. Positive values of
DH* indicate the presence of variation
The higher activity of Ni complex can be clarified by the

N.E.A. El-Gamel, K.A. Ali / Journal of Molecular Structure 1147 (2017) 167e176
175
Fig. 6. Thermogravimetric analysis (TG/DTA) of ligand and complexes (1) & (2).
fundamental response of each microorganism according to the
difference in the chemical composition of each cell membrane
besides the different function of the coordinated metal ions in the
biological system. It is worth to noted that, the chelation effect may
change the polarity of the metal ion, which would accelerate and
enhance the permeability and the penetration of the central metal
atom through the lipid layers of cell membrane [38,44]. It was
recorded that the dipole moment of Ni complex (1) (9.69 Debye) is
higher than of Mn complex (2) (7.39 Debye); thus the activity Ni
complex (1) is enhanced due to its higher polarity; this conse-
quently would increase its hydrophilic character and would favour
its penetration through the lipid layers of the bacterial/fungal
membrane, leading to damage and rapid cell death [10e12,46].
3
The Minimum Inhibitory Concentrations (MIC,
m
g/cm ) of the Ni
complex was recorded; the results show that the nickel complex (1)
displayed comparable antibacterial activity against Staphylococcus
aureus (Gram-positive bacteria); MIC value is 30
sup>, which is quite near to the value of the reference antibacterial
ampicillin (25 g/cm³). Additionally, the MIC value is
found to be 13 g/cm³ against the fungus Candida
albicans, this value is in the range of the reference antifungal clo-
mg/cm<sup>3</
m
m
3
trimazole (12.5 mg/cm ). Thus, conceptually, Ni complex (1) could
Fig. 7. Biological activity of Ni(II) (1) and Mn(II) (2) complexes against the tested
bacteria and fungi with evaluation of the standard effects of antibacterial and anti-
fungal agents.
be used as an alternative compound against the viability of Staph-
ylococcus aureus and Candida albicans to overcome the resistance to
existing antibiotics and providing promising insight for potential
class of new fungicides and antibacterial agents. The potential
antimicrobial activity of Ni(II) and Mn(II) complexes showed an
agreement and similarity comparing to other previous published
articles [10,47e49], therefore it is important to highlight the con-
tinuity of this contribution.

1
76
N.E.A. El-Gamel, K.A. Ali / Journal of Molecular Structure 1147 (2017) 167e176
[13] I. Galasko, J. South Afr. Chem. Inst. 22 (1969) 121e125.
The potential of this study lies in the applicability of using these
[
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14] M. Zahedifar, H. Sheibani, Chem. Heterocycl. Compd. 52 (2016) 41e44.
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caused by C. albicans and S. aureus like oropharyngeal candidiasis,
vulvovaginal candidiasis, and invasive candidiasis [50] and super-
ficial skin infections in humans, Infections of organs (lungs, heart,
bones, veins, blood vessels, brain; respectively [51].
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original literature references for NBO can also be cited The original literature
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-yl)-3-oxo-3-phenylpropanamide ligand has been carried out
2
with the goal of designing new structural compounds serving as
potent antimicrobial agents. Structural characterization has been
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