# Synthesis, Characterization, and Biological Activity ofN1-Methyl-2-(1H-1,2,3-Benzotriazol-1-y1)-3-Oxobutan-  ethioamide

Authors: Nouria A. Al-Awadi; Nadia M. Shuaib; Alaa Abbas; Ahmed A. El-Sherif; Ali El-Dissouky; Esmaeil Al-Saleh
Journal: Bioinorganic Chemistry and Applications
 (2008)

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## Abstract

A new series ofZn2+, Cu2+, Ni2+,  and Co2+ complexes of N1-methyl-2-(1H-1,2,3-benzotriazol-1-yl)-3-oxobutanethioamide (MBOBT), HL, has been synthesized and characterized by different spectral and  magnetic measurements and elemental analysis. IR spectral data indicates that (MBOBT) exists only in the thione form in the solid state while
13C NMR spectrum indicates its existence in thione and thiole tautomeric forms. The IR spectra of all complexes indicate that (MBOBT) acts as a monobasic bidentate ligand coordinating to the metal(II) ions via the keto-oxygen and thiolato-sulphur
atoms. The electronic spectral studies showed that (MBOBT) bonded to all metal ions through sulphur and nitrogen atoms based on the positions and intensity of their charge transfer bands. Furthermore, the spectra reflect four coordinate tetrahedral zinc(II),  tetragonally distorted copper(II), square planar nickel(II),
and cobalt(II) complexes. Thermal decomposition study of the complexes was monitored by TG and
DTG analyses under N2 atmosphere. The decomposition course and steps were analyzed and the activation parameters of the nonisothermal decomposition are determined. The isolated metal chelates have been screened for their antimicrobial activities and the findings have been reported and discussed in relation to their structures.


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## Body

## 1. Introduction

Compounds containing triazoles have
attracted much interest because of their biological applications [1–4].
Furthermore, triazoles appear frequently in the structures of various natural
products [5]. Triazole containing compounds appear in many metabolic products
of fungi and primitive marine animals. Many triazoles having different
functionalities are used as dyes and as photographic chemicals [6]. The
coordination chemistry of triazole and
benzotriazole derivatives was studied due to their importance in industry,
agriculture and their biological activity. The mercapto group often coordinated
to metal ions in many biological molecules [7] and information about the
relative reactivity of the coordinated mercapto group might give insight into
the specific reactivity of active sites
in some metalloproteins. On the other hand, some of the transition metals
present in trace quantities are essential elements for biological systems. In
view of the above facts and in continuation of our interest in studying the
ligating behavior of such compounds [8–11],
we aim to (i) synthesize and characterize the solid
complexes of the newly ligand containing both the triazole and thioamide moieties,  N1-methyl-2-(1H-1,2,3-benzotriazol-1-yl)-3-oxobutanethioamide
(MBOBT), HL, I with Zn2+, Cu2+, Ni2+, and
Co2+, (ii) study their thermal decomposition characteristics and determine
the different thermodynamic parameters, and (iii) investigate their
antimicrobial effects towards some Gram-positive and Gram-negative bacteria.


## 2. Experimental

### 2.1. Materials and Reagents

All chemicals were reagent grade quality
obtained from BDH and Aldrich Chemical Companies and used as received.


### 2.2. Synthesis of MBOBT

The organic ligand was prepared according to
the previously reported method [12].


### 2.3. Synthesis of Metal Complexes

The complexes are synthesized by the
general method, namely, a solution of hydrated metal(II) acetate(0.001 mol;
0.22, 0.19, 0.25, and 0.25 g of Zn (OAc)2⋅2H2O,
Cu(OAc)2⋅H2O, Ni(OAc)2⋅4H2O,
Co(OAc)2⋅4H2O,
resp.)in EtOH (30 mL), Cu(OAc)2.H2O was dissolved in MeOH,  and (0.0022 mol, 0.53 g)
in EtOH (25 mL) followed by the addition of 3–5 mL triethylamine (TEA). The
reaction mixture was refluxed for 2-3 hours on a water bath and then cooled to the room temperature. The
solid product in each case was filtered off, washed several times with EtOH, Et2O,
and dried in vacuum over P4O10.


### 2.4. Screening for Antibacterial
Activity

The synthesized MBOBT and its four
metal(II) complexes were  screened in vitro for their antibacterial activity
against five Gram-positive (Staphylococcus aureus,Staphylococcus
hominis, Bacillussp1, Bacillussp2, andBacillussp3
)  and three Gram-negative
(Escherichia coli, Salmonellasp1andSalmonellasp2
) bacterial strains using gel diffusion and respirometric method. The gel
diffusion method was used as previously described [13]. Bacterial cultures were
grown overnight on nutrient agar (NA) plates. Bacterial biomass was suspended
in 0.9% saline and adjusted to an optical density (OD) of 0.02 at λ 600 nm.
Bacterial suspensions were spread on the NA plates using sterile cotton swaps.
Uniform wells were created in the NA plates using a cork-borer (6 mm). Synthesized chemicals (dissolved in ethanol) were transferred (100 μL, 0.1 mg) into the wells and
ethanol was used as control. Plates were incubated for 24 hours at 30∘C and
the diameter of inhibition zones around the wells was measured in centimeter
(cm). Each test was conducted in triplicate and the mean with standard
deviation was calculated. The inhibitory effects of synthesized MBOBT and its
four metal(II) complexes in ethanol as solvent on bacterial respiration were
also investigated using the method of Al-Saleh and Obuekwe [14]. Synthesized
chemicals (0.5 and 1 mg) were transferred to sterile bottles containing 49 mL
nutrient broth and bacterial culture (1 mL of overnight culture, OD 1 at λ 600 nm). Bottles were connected to respirometer (Micro-Oxymax Columbus Instruments)
and incubated in a shaking water bath at 30∘C. Bottles with sterile
nutrient broth were used as control. Experiments were conducted in triplicates
and the amount of carbon dioxide evolved was plotted against time. In order to
clarify any participating role of EtOH in the biological screening, separate
studies were carried out with the solutions without the complexes and they
showed less or no activity against any bacteria.Physical Measurements and Analysis
CHNS analysis was obtained using LECO-CHNS 932 Analyzer.
FT-IR spectra were recorded as KBr discs with Schimadzu 2000 FT-IR
spectrophotometer. Electronic spectra were accomplished by Carry Varian 5
UV/Vis spectrophotometer. The room temperature magnetic susceptibility
measurements for the complexes were determined by the Gouy balance using
Hg[Co(NCS)4] as a calibrant. Thermal analysis measurement was
performed by using a dynamic nitrogen atmosphere with a TGA-50 Shimadzu
thermogravimetric analyzer at a flow rate of 50 mL⋅min−1⁡. The
heating rate was 10∘C⋅min−1⁡ and the sample sizes ranged in mass from 6 to 8 mg. 1H
NMR was determined on a Bruker DPX 400 MHz superconducting spectrometer in CDCl3
and DMSO-d6 as solvents and using TMS as internal standard.


## 2.1. Materials and Reagents

All chemicals were reagent grade quality
obtained from BDH and Aldrich Chemical Companies and used as received.


## 2.2. Synthesis of MBOBT

The organic ligand was prepared according to
the previously reported method [12].


## 2.3. Synthesis of Metal Complexes

The complexes are synthesized by the
general method, namely, a solution of hydrated metal(II) acetate(0.001 mol;
0.22, 0.19, 0.25, and 0.25 g of Zn (OAc)2⋅2H2O,
Cu(OAc)2⋅H2O, Ni(OAc)2⋅4H2O,
Co(OAc)2⋅4H2O,
resp.)in EtOH (30 mL), Cu(OAc)2.H2O was dissolved in MeOH,  and (0.0022 mol, 0.53 g)
in EtOH (25 mL) followed by the addition of 3–5 mL triethylamine (TEA). The
reaction mixture was refluxed for 2-3 hours on a water bath and then cooled to the room temperature. The
solid product in each case was filtered off, washed several times with EtOH, Et2O,
and dried in vacuum over P4O10.


## 2.4. Screening for Antibacterial
Activity

The synthesized MBOBT and its four
metal(II) complexes were  screened in vitro for their antibacterial activity
against five Gram-positive (Staphylococcus aureus,Staphylococcus
hominis, Bacillussp1, Bacillussp2, andBacillussp3
)  and three Gram-negative
(Escherichia coli, Salmonellasp1andSalmonellasp2
) bacterial strains using gel diffusion and respirometric method. The gel
diffusion method was used as previously described [13]. Bacterial cultures were
grown overnight on nutrient agar (NA) plates. Bacterial biomass was suspended
in 0.9% saline and adjusted to an optical density (OD) of 0.02 at λ 600 nm.
Bacterial suspensions were spread on the NA plates using sterile cotton swaps.
Uniform wells were created in the NA plates using a cork-borer (6 mm). Synthesized chemicals (dissolved in ethanol) were transferred (100 μL, 0.1 mg) into the wells and
ethanol was used as control. Plates were incubated for 24 hours at 30∘C and
the diameter of inhibition zones around the wells was measured in centimeter
(cm). Each test was conducted in triplicate and the mean with standard
deviation was calculated. The inhibitory effects of synthesized MBOBT and its
four metal(II) complexes in ethanol as solvent on bacterial respiration were
also investigated using the method of Al-Saleh and Obuekwe [14]. Synthesized
chemicals (0.5 and 1 mg) were transferred to sterile bottles containing 49 mL
nutrient broth and bacterial culture (1 mL of overnight culture, OD 1 at λ 600 nm). Bottles were connected to respirometer (Micro-Oxymax Columbus Instruments)
and incubated in a shaking water bath at 30∘C. Bottles with sterile
nutrient broth were used as control. Experiments were conducted in triplicates
and the amount of carbon dioxide evolved was plotted against time. In order to
clarify any participating role of EtOH in the biological screening, separate
studies were carried out with the solutions without the complexes and they
showed less or no activity against any bacteria.Physical Measurements and Analysis
CHNS analysis was obtained using LECO-CHNS 932 Analyzer.
FT-IR spectra were recorded as KBr discs with Schimadzu 2000 FT-IR
spectrophotometer. Electronic spectra were accomplished by Carry Varian 5
UV/Vis spectrophotometer. The room temperature magnetic susceptibility
measurements for the complexes were determined by the Gouy balance using
Hg[Co(NCS)4] as a calibrant. Thermal analysis measurement was
performed by using a dynamic nitrogen atmosphere with a TGA-50 Shimadzu
thermogravimetric analyzer at a flow rate of 50 mL⋅min−1⁡. The
heating rate was 10∘C⋅min−1⁡ and the sample sizes ranged in mass from 6 to 8 mg. 1H
NMR was determined on a Bruker DPX 400 MHz superconducting spectrometer in CDCl3
and DMSO-d6 as solvents and using TMS as internal standard.


## 3. Results and Discussion

### 3.1. General

The reaction of (MBOBT) with metal
ions under stirring and different mole ratios gave the complexes presented in
Table1 and their formulation is based on the
obtained elemental analyses. The complexes are air stable, insoluble in the
most organic solvents and water but freely soluble in DMF and DMSO. The
complexes have higher melting points than their corresponding ligands
indicating that they are thermally stable. This could be attributed to the
formation of chelate rings and/or increased in conjugation due to complexation.Table 1
Elemental analysis [% found (% calculated)],
color, and the room-temperature effective magnetic moments (B.M.) of MBOBT and its
metal(II) complexes.
CompoundColorμeffC(%)H(%)N(%)S(%)HL,
C11H12N4SOBuff—53.0 (53.2)4.7 (4.8)22.3 (22.6)12.6 (12.9)[L2Zn]
⋅ H2O,
C22H24N8S2O3ZnBuffDiamag45.3 (45.6)4.0 (4.2)19.3 (19.4)11.0 (11.1)[L2Cu], C22H22N8S2O2CuLight
blue1.8247.1 (47.4)4.2 (4.0)20.1 (20.0)11.1 (11.4)[
L2Ni
], C22H22N8S2O2NiDark
blueDiamag47.3 (47.6)4.1 (4.0)19.9 (20.2)11.3 (11.5)[L2Co], C22H22N8S2O2CoDark
red2.7047.7 (47.8)4.2 (4.0)19.8 (20.2)11.7 (11.6)


### 3.2. Characterization of the MBOBT and
Its Solid Complexes

#### 3.2.1. NMR and IR Spectra of MBOBT and Its Complexes

The 13C NMR spectrum ofMBOBT in d6-DMSO was recorded. Despite expecting signals for only
10 carbons, twenty carbon signals appeared with the spectra indicating that at
least in DMSO, the molecule exists as an equilibrium mixture of two forms (I A and
I B). The existence of a signal at δ 76.16 ppm characteristic of an sp3 carbon indicated clearly that one of these two forms is A. In the C=O region
two carbonyl carbons at δ 197.9 and 192.3 ppm are detected indicating the presence
of C=O in both forms. The spectrum
exhibits only one C=S signal at δ 197.98 ppm (should
be appeared at δ 173 ppm). Furthermore, the spectrum displays a signal at δ
173.4 ppm characteristic of an sp2 carbon in accordance with the
assumption of the second form B.The infrared spectra of MBOBT
and its different complexes are recorded as KBr discs and main bands with their
tentative assignments given in Table2. The spectrum of MBOBT does not show ν(SH) band at 2600–2500 cm−1, in which this stretching frequency is generally
expected and is therefore mainly in the
thioketo form [15]. The spectrum of MBOBT displays four bands
1499,
1375, 1073, and 836 cm−1 assigned to the thioamide bands, namely I, II, III, and IV, contains a
thioamide group (HNC=S), and has contribution from δ(C–H) + δ(N–H), ν(C=S)  +  ν(C=N)  +  ν(C–H), ν(C–N)  +  ν(C–S),
and ν(C…S),
respectively [16, 17]. These thioamide bands III and
IV are strongly shifted to lower wavenumbers in the spectra of all complexes
supporting sulphur donation and deprotonation of the ligand as well.
Furthermore, the thioamide bands I and II are not greatly affected by
complexation suggesting the nonbonding nature of the nitrogen to the metal ion.
The spectrum of MBOBT displays only a weak C=O absorption at 1644 cm−1 and medium-strong band at 3280 cm−1 due to υ(NH). Shifting position and
decreasing intensity of the υ(C=O) and shifting of the
υ(NH) to longer
wavelength and  increasing intensity may
be due to hydrogen bonding formation between these two groups. As expected, CH
stretches for C–H linked to sp2 and sp3 carbon  appeared
at 3045 and 2947 cm−1, respectively.Table 2
Main
IR (ύ, cm−1)
bands for MBOBT
and its metal(II) complexes.  (w  =  week, m  =  medium, s = strong.)
Compoundυ(OH)υ(NH)υ(C=O)υ(CH3)υ(thioamide)IIIIIIIVHL,
C11H12N4SO—3280s1644w2947w1499vs1375m1073m836s3045w[L2Zn] ⋅ H2O,
C22H24N8S2O3Zn3433m3230s1600w2996w1497m1383m1046m720m3073w[L2Cu], C22H22N8S2O2Cu—3228s1614w2927w1499s1388s1050m734m3084w[L2], C22H22N8S2O2Ni—3328s1613w2929w1501m1391s1042m723m[L2Co], C22H22N8S2O2Co—3230s1616w2992w1496m1379s1039w718m3072wAll these data suggest the presence
of the free MBOBT in the form A in the solid state. This band is red shifted by
ca. 18–44cm−1   upon complex formation supporting the bonding of
oxygen to the metal ion. Accordingly, BMMB acts as a monobasic bidentate ligand
coordinated to the metal ions through the deprotonated thiolo-sulphur and
keto-oxygen atoms [18, 19].


#### 3.2.2. Electronic Spectra and Magnetic
Studies

The
electronic spectra of the complexes, Table3, show intense bands at 26700–28600
and 29300–29700 cm−1 attributable to the intra-ligand O–M(II)
transitions suggesting the bonding of the ligand oxygen to the metal ion. The
spectra also exhibit a strong band at 22300–24700 cm−1 characteristic of S–M(II) LMCT transition and further support the bonding of
the ligand to the metal ion via a
sulphur atom.Table 3
Electronic spectral data (cm−1) for MBOBT complexes.
CompoundIntraligand
and CT transitionsd-d
transitions[L2Zn] ⋅ H2O,
C22H24N8S2O3Zn29280,
26850, 23700—[L2Cu], C22H22N8S2O2Cu26200—[L2Ni], C22H22N8S2O2Ni29700,
28600, 2450024200,19950, 16890[L2Co], C22H22N8S2O2Co29620,
26700, 247008500, 19960The electronic
spectrum of [L2Zn] ⋅ H2O shows intense bands at 29280, 26850, and 23700 cm−1
which
are assigned to the intraligands O → Zn(II) and S
→ Zn(II) LMCT, respectively. These
spectral features indicate the bonding of BMMB to the Zn(II) via oxygen and
sulphur atoms. The spectrum shows no bands in the region below 23000 cm−1 which is in accordance with the d10 electronic configuration of
Zn(II).Copper(II)
complex [L2Cu] gives a room temperature magnetic moment value of
1.78 B.M. characteristic of magnetically diluted copper(II) species. Its
electronic spectrum displays only an intense band at 26200 cm−1 which
is attributed to the intraligand (ligand localized) and LMCT transitions and
characteristic of a tetragonally distorted copper(II) complexes.The
electronic spectrum of [L2Ni] displays bands at, 16890, 19950, and
24200 cm−1  assignable to  1A1gυ→1A2g, (υ1), 1A1g→1B1g(υ2),   and 1A1g→1Eg(υ3) transitions, respectively, characteristic of
square planar nickel(II) complexes. The first two bands are pure d-d
transitions while theυ3 band obviously enveloped by a strong
CT transition. The assumed square planar geometry for this complex is confirmed
from the value of its room temperature magnetic moment of zero.The room
temperature magnetic moment of [L2Co] of 2.70 B.M. is more than that
of low spin octahedral and lower than the values characteristic of tetrahedral
cobalt(II) complexes. Furthermore, these values are similar to that reported
for the square planar cobalt(II) complexes [20, 21]. The electronic spectrum of
the complex exhibits two bands at 8500 and 19960 cm−1 characteristic
of square planar cobalt(II) complexes with a transition involving nonbonding
rather antibonding orbitals. In a strong field, the ground state is probably  2A1g with the configuration of eg4b2g2a1g1.


#### 3.2.3. Thermal Analysis

The thermogravimetric (TG) and the
derivative thermogravimetric (DTG) plots of the complexes in the 25–1200∘C
range under N2  are shown in Figures 1–4. Their stepwise thermal degradation data are
given in Table 4. All complexes show two-stage mass loss
except [L2Zn]⋅H2O shows three decomposition steps.Table 4
Stepwise
thermal degradation data obtained from TGA curves for the metal complexes.
ComplexMolar massTG range
(C∘)DTGmax (C∘)Weight lossPredicated
intermediates and final productsMetallic residue (calcd. %) foundCalcd.Found[L2Zn] ⋅ H2O,
C22H24N8S2O3Zn577.332–120723.12.9H2OZnS136–32826149.849.22-BTA(16.5) 17.4[L2Cu], C22H22N8S2O2Cu557.5158–31621651.651.22-BTACuS449–54950031.230.9SO2  +  L1(17.2) 17.9[L2Ni],
C22H22N8S2O2Ni552.732–99781.61.51/2 H2O222–33129052.151.82-BTANiS414–60853131.431.1SO2 + L1(16.4) 17.1[L2Co],
C22H22N8S2O2Co552.9222–33129052.0852.72-BTACoS414–60853131.429.9SO2 + L1(16.5) 17.4222–33129052.0852.72-BTACoSBTA  =  benzotriazole ring, L1=  C6H10N2.Figure 1
TG and DTG plots of [L2Zn]⋅H2O.Scheme 1Figure 2
TG and DTG plots of
[L2Cu].Figure 3
TG and DTG plots of [L2Ni].Figure 4
TG and DTG plots of [L2Co].The
TG and DTG curves of [L2Zn]⋅H2O are shown in Figure 1. The
TGA curve of this complex shows three stages of decomposition within the
temperature range (32–654∘C). The first step of decomposition within the
temperature range (32–120∘C) corresponds to
the loss of water molecule of hydration with mass loss of 2.9% (calcd. 3.1%). The
second step (136–328∘C)
corresponds to the loss of two benzotriazole (BTA) moities and two acetylene
molecules (mass loss 49.2%; calcd. 49.8%). The third step (545–654∘C)
corresponds to the loss of SO2 and  L1 molecules (mass loss
30.1%; calcd. 29.8%). The energies of activation were 43.73, 37.1 and 24.4 kJ mol−1 for the first, second, and third steps, respectively. The
total mass loss up to 654∘C is in agreement with the formation
of ZnS as the final residue (TG 16.1%, calcd. 16.4%).The
thermogram given in Figure2 of [L2Cu] exhibits two significant
thermal events within the temperature range (158–549∘C).
The first step of decomposition within the
temperature range (158–316) corresponds to the loss of two BTA and two
acetylene molecules with a mass loss 51.2% (calcd. 51.6%). The second step
(449–549∘C) corresponds to the loss of
SO2 and  L1  molecules (mass loss 30.9%; calcd. 31.2%). The energies of
activation were 73.83 and 26.36 kJ mol−1 for the first and second
steps, respectively. The total mass loss up to 549∘C is in agreement
with the formation of CuS as the final residue (TG 17.2%, calcd. 17.9%).The
TG and DTG curves of [L2Ni] are shown in Figure 3. The TGA curve shows
two stages of decomposition within the temperature range (222–608∘C). The first step of decomposition within the
temperature range (222–331∘C) corresponds to the loss of
two
BTA and two acetylene molecules with a mass
loss of 52.1% (calcd. 51.8%). The second step (414–608∘C) corresponds to
the loss of L1  molecule (mass loss 31.4%; calcd. 31.1%). The
energies of activation were 64.3 and 16.7 kJ mol−1 for the first and
second steps, respectively. The total mass loss up to 608∘C is in agreement
with the formation of NiS as the final residue (TG 16.4%, calcd. 17.1%).The
[L2Co] complex is thermally stable up to 670∘C; see
Figure 4.
From the TG curve it appears that the complex decomposes in two stages over the
temperature range 160–670∘C. The first
decomposition occurs between 160–237∘C with mass loss of (calcd.
51.6%) and the second decomposition starts at 237∘C and ends at 670∘C with a 31.1% mass
loss (calcd. 31.4%). The first step of decomposition corresponds to the loss of
two BTA and two acetylene molecules
while  the second step corresponds to the loss of SO2 and   L1
molecules. The energies  of activation were 59.5 and 14.0 kJ mol−1  for the first and second steps, respectively.


#### 3.2.4. Kinetic Data for the
Decomposition of Complexes

The
thermodynamic parameters of
decomposition processes of complexes, namely, activation energy (Ea),
enthalpy (ΔH*),
entropy (ΔS*),
and Gibbs free energy change of (ΔG*)
were evaluated graphically by employing the Coats-Redfern  method [22, 23]. This method, reviewed by Johnson and Gallagher [23] as
an integral method assuming various orders of reaction and comparing the
linearity in each case to select the correct order by using(1)log⁡[1−(1−α)1−nT2(1−n)]=log⁡[ARθEa(1−2RTEa)]−Ea2.303RTforn≠1,log⁡{−log⁡(1−α)T2}=log⁡[ARθEa(1−2RTEa)]−Ea2.303RTforn=1, where α is the
fraction of sample decomposed at time t, T  is the derivative peak temperature,
A is the frequency factor, Ea is the activation energy, R is the gas constant, θ is the heating rate, and  (1 − (2RT/Ea)) ≅ 1. A plot of log{− log (1−a)/T2} versus 1/T
gives a slope from which the Ea was calculated and A (Arrhenius
factor) was determined from the intercept. Trials of these plots were made by
assuming the orders 0, 1/2, and 1 and the best plot was obtained for the first
order. The entropy of activation was calculated using [24](2)ΔS*=2.303R[log⁡(AhkT)], where h and k  stand for the Planck and
Boltzmann constants, respectively, and T is the peak temperature from the DTG
curve. The free energy of activation  ΔG* and
the enthalpy of activation  ΔH* are
calculated using (4), (3)ΔH*=Ea−RT,ΔG*=ΔH*−TΔS*. The kinetic data obtained from the
nonisothermal decomposition of the complexes are given in Table 5.Table 5
The kinetic parameters for the nonisothermal
decomposition of the complexes.
Complex/range (C∘)T(a)Ea (KJ mol−1)A (S−1)ΔH*
(kJ mol−1)ΔS*
(JK−1mol−1)ΔG*
(kJ mol−1)[L2Cu
]90–13421673.831.57E + 0469.76−169.14152.47220–26050026.364.0E− 0319.93−299.09251.13[L2Ni]222–33929064.353.7E + 0269.70−201.45173.08430–60853116.711.5E− 0310.03−307.60257.30[L2Co]160–32722759.532.29E + 0255.37−204.40157.60327–67045214.042.67E− 048.01−321.10240.80[L2Zn] ⋅ H2O32–1207243.731.91E + 0340.87−183.70104.06153–33626137.084.15E− 0132.64−257.50170.12544–66259524.401.29E− 0317.23−309.50285.87(a)The peak
temperature from the DTG curve.The activation energy of the
complexes is expected to increase with decreasing metal ion radius [25, 26].
The smaller size of metal ions permits a closer approach of the ligand.Hence, theΔE*
value in
the first stages for the Cu(II) complex is higher than those of Ni(II), Co(II),
and Zn(II) complexes [27–29]. The calculated ΔE*
values using
Coats-Redfern method for the first-stage decomposition of the complexes are
found to beΔECu*=  73.8 kJ mol−1 > ΔENi* =  64.3 kJ mol−1 >  ΔECo*  =  59.5 kJ mol−1 which is in accordance with rCu(II)= 70 pm  <  rNi(II) =  72 pm  <  rCo(II) =  74 pm.The same decomposition kinetics is
also true for theΔE* values of the second stage decomposition which
was found to be in the following order:ΔECu* =  26.3 kJ mol−1> ΔENi* =  16.7 kJ mol−1 >  ΔECo* = 14.0 kJ mol−1.The negative values ofΔS*, see Table 5, indicate that the reaction rates are slower than normal [30] which is
consistent with the results reported previously [31]. Furthermore, these data
indicate that the activated complexes have more ordered structure than the
reactants [29–31].


#### 3.2.5. Biological Activity

The antibacterial activity of MBOBT
and its metal(II) complexes are given inTables 6 and 7 and the average of three experimental data for [L2Zn]⋅H2O
and [L2Cu] are shown in Figures 5–15. The results show that (i) the complexes exhibit inhibitory effects
towards the activity of gram-positive and gram-negative bacteria in contrast to
the parent organic ligand which is biologically inactive under the experimental
conditions, (ii) all complexes are inactive towards Salmonellasp2 and only [L2Cu] is active towards
Staphylococcus aurous, and (iii) copper(II) complex has a wide spectrum with respect
to the studied bacteria. As previously reported, the metal salts do not exhibit
antimicrobial activity [32–36]. The biological activity of the metal complexes
is governed by the following factors [36]: (i) the chelate effect of the ligands,
(ii) the nature of the donor atoms, (iii) the total charge on the complex ion, (iv)
the nature of the metal ion, (v) the nature of the counter ions that neutralize
the complex, and (vi) the geometrical structure of the complex [35].
Furthermore, chelation reduces the polarity of the metal ion because of partial
sharing of its positive charge with the donor groups and possibly the π-electron
delocalization within the whole chelate ring system that is formed during
coordination [33]. These factors increase the lipophilic nature of the central
metal atom and hence increasing the hydrophobic character and liposolubility of
the molecule favoring its permeation through the lipid bilayer of the bacterial
membrane. This enhances the rate of uptake/entrance and thus the antibacterial
activity of the testing compounds. Accordingly, the antimicrobial activity of
the four complexes can be referred to the increase of their lipophilic
character which in turn deactivates enzymes responsible for respiratory
processes and probably other cellular enzymes, which play a vital role in
various metabolic pathways of the tested bacteria. Also it is proposed that the
action of the toxicant is the denaturation of one or more proteins of the cell
and this impairs normal cellular process. According to the data given in Tables
6 and 7, the antimicrobial activity can be ordered as [L2Cu]
> [L2Zn]⋅H2O >
[L2Ni] > [L2Co],
suggesting that the lipophilic behavior
increases in the same order. Since all complexes (i) have the same
donating atoms which are S/O with the same coordination number (C.N. for each
is 4), (ii) have the same chelate effect (all form two 6-membered chelating
rings), (iii) are neutral and there are no counter ions, and (iv) have the same
oxidation number in their complexes (M2+), therefore, the more
effective factors are the geometrical shape and the nature of the central
atoms. According to the spectral and magnetic studies, (i) copper has a
tetragonal distortion (distorted to a tetrahedral geometry); (ii) cobalt and
nickel have a square planar; (iii) zinc is associated with a tetrahedral
geometry. Therefore, the higher antimicrobial activity can be referred to their
similar structure which is the tetrahedral. This structure increases the
lipophilicity of the central atom by decreasing the effective nuclear charge
(polarity) of the Zn(II) and Cu(II) more than the square planar structure of
Co(II) and Ni(II). The higher antimicrobial activity of copper(II) complex
relative to the zinc(II) complex may be referred to the presence of water
molecule in the formula of the later complex, also copper(II) may form stronger
copper(II)-ligand bond than Zn(II)-ligand bond and this in turn increases the lipophilic
character of copper(II) complex than zinc(II) complex. The redox activity of
copper compared to zinc, which is redox neutral, may be taken as an additional
reason for the higher activity of copper relative to zinc complex.Table 6
Effect of MBOBT and its complexes on the
respiration of bacteria. (Results represent percent
inhibition of bacterial respiration caused by the addition of 0.5 and 1.0 mg
of the test compound. Results are the mean of
three independent analyses with standard
deviations.)
CompoundBacteriaStaphylococcus aureusStaphylococcus hominisBacillussp1Bacillussp2Bacillussp3Escherichia coliSalmonellasp1Salmonellasp2[L2Zn] ⋅ H2OAmount (mg)0.5nilnil94.7±6.353.2±3.584.3±5.6nil40.7±2.5nil1.0nilnil93.1±6.394.5±6.386.1±5.7nil79.1±5.3nil[L2Cu]0.589.6±6.087.3±5.884.6±5.690.3±6.088.5±5.892.6±6.267.7±4.5nil1.094.5±6.391.0±6.095.1±6.566.7±4.487.9±6.093.7±6.474.8±5.0nil[L2Ni]0.5nilnil68.3±4.673.5±4.944.6±3.0nilnilnil1.0nil16.5±1.089.5±6.090.8±6.067.2±4.5nilnilnil[L2Co]0.5nilnil26.8±1.736.5±2.433.2±2.2nilnilnil1.0nilnil69.9±4.778.9±5.370.1±4.6nilnilnilBMMB0.5nilnilnilnilnilnilnilnil1.0nilnilnilnilnilnilnilnilTable 7
Antimicrobial results (zone of inhibition, diameter in cm) of MBOBT and its
complexes using gel-diffusion
method.
CompoundBacteriaStaphylococcus aureusStaphylococcus hominisBacillussp1Bacillussp2Bacillussp3Escherichia coliSalmonellasp1Salmonellasp2[L2Z] ⋅ H2Onilnil3 ±  0.11.8±0.051.5±0.05nil1.2±0.04nil[L2Cu]1 ±  0.031.2±0.043.2±0.12  ± 0.062.1±0.061.2±0.041.9±0.06nil[L2Ni]1.2±0.041 ±  0.030.9±0.030.9±0.031 ±  0.04nil1.4±0.05nil[L2Co]0.9±0.031 ±  0.031.2±0.041.2±0.040.9±0.030.9±0.03nilnilBMMBnilnilnilnilnilnilnilnilFigure 5
Effect of
[L2Cu] on the respiration of  Bacillussp1.Figure 6
Effect
of [L2Cu] on the respiration of Bacillussp2.Figure 7
Effect
of [L2Cu] on the respiration of Bacillussp3.Figure 8
Effect
of [L2Cu] on the respiration of
E. coli.Figure 9
Effect
of [L2Cu] on the respiration of Salmonellasp1.Figure 10
Effect
of [L2Cu] on the respiration of Staphylococcus aurous.Figure 11
Effect of [L2Cu]
on the respiration of Staphylococcus hominis.Figure 12
Effect
of [L2Zn]⋅H2O on the respiration of Salmonellasp1.Figure 13
Effect
of [L2Zn]⋅H2O on the respiration of Bacillussp1.Figure 14
Effect
of [L2Zn]⋅H2O on the respiration of Bacillussp2.Figure 15
Effect
of [L2Zn]⋅H2O on the respiration of Bacillussp3.


## 3.1. General

The reaction of (MBOBT) with metal
ions under stirring and different mole ratios gave the complexes presented in
Table1 and their formulation is based on the
obtained elemental analyses. The complexes are air stable, insoluble in the
most organic solvents and water but freely soluble in DMF and DMSO. The
complexes have higher melting points than their corresponding ligands
indicating that they are thermally stable. This could be attributed to the
formation of chelate rings and/or increased in conjugation due to complexation.Table 1
Elemental analysis [% found (% calculated)],
color, and the room-temperature effective magnetic moments (B.M.) of MBOBT and its
metal(II) complexes.
CompoundColorμeffC(%)H(%)N(%)S(%)HL,
C11H12N4SOBuff—53.0 (53.2)4.7 (4.8)22.3 (22.6)12.6 (12.9)[L2Zn]
⋅ H2O,
C22H24N8S2O3ZnBuffDiamag45.3 (45.6)4.0 (4.2)19.3 (19.4)11.0 (11.1)[L2Cu], C22H22N8S2O2CuLight
blue1.8247.1 (47.4)4.2 (4.0)20.1 (20.0)11.1 (11.4)[
L2Ni
], C22H22N8S2O2NiDark
blueDiamag47.3 (47.6)4.1 (4.0)19.9 (20.2)11.3 (11.5)[L2Co], C22H22N8S2O2CoDark
red2.7047.7 (47.8)4.2 (4.0)19.8 (20.2)11.7 (11.6)


## 3.2. Characterization of the MBOBT and
Its Solid Complexes

### 3.2.1. NMR and IR Spectra of MBOBT and Its Complexes

The 13C NMR spectrum ofMBOBT in d6-DMSO was recorded. Despite expecting signals for only
10 carbons, twenty carbon signals appeared with the spectra indicating that at
least in DMSO, the molecule exists as an equilibrium mixture of two forms (I A and
I B). The existence of a signal at δ 76.16 ppm characteristic of an sp3 carbon indicated clearly that one of these two forms is A. In the C=O region
two carbonyl carbons at δ 197.9 and 192.3 ppm are detected indicating the presence
of C=O in both forms. The spectrum
exhibits only one C=S signal at δ 197.98 ppm (should
be appeared at δ 173 ppm). Furthermore, the spectrum displays a signal at δ
173.4 ppm characteristic of an sp2 carbon in accordance with the
assumption of the second form B.The infrared spectra of MBOBT
and its different complexes are recorded as KBr discs and main bands with their
tentative assignments given in Table2. The spectrum of MBOBT does not show ν(SH) band at 2600–2500 cm−1, in which this stretching frequency is generally
expected and is therefore mainly in the
thioketo form [15]. The spectrum of MBOBT displays four bands
1499,
1375, 1073, and 836 cm−1 assigned to the thioamide bands, namely I, II, III, and IV, contains a
thioamide group (HNC=S), and has contribution from δ(C–H) + δ(N–H), ν(C=S)  +  ν(C=N)  +  ν(C–H), ν(C–N)  +  ν(C–S),
and ν(C…S),
respectively [16, 17]. These thioamide bands III and
IV are strongly shifted to lower wavenumbers in the spectra of all complexes
supporting sulphur donation and deprotonation of the ligand as well.
Furthermore, the thioamide bands I and II are not greatly affected by
complexation suggesting the nonbonding nature of the nitrogen to the metal ion.
The spectrum of MBOBT displays only a weak C=O absorption at 1644 cm−1 and medium-strong band at 3280 cm−1 due to υ(NH). Shifting position and
decreasing intensity of the υ(C=O) and shifting of the
υ(NH) to longer
wavelength and  increasing intensity may
be due to hydrogen bonding formation between these two groups. As expected, CH
stretches for C–H linked to sp2 and sp3 carbon  appeared
at 3045 and 2947 cm−1, respectively.Table 2
Main
IR (ύ, cm−1)
bands for MBOBT
and its metal(II) complexes.  (w  =  week, m  =  medium, s = strong.)
Compoundυ(OH)υ(NH)υ(C=O)υ(CH3)υ(thioamide)IIIIIIIVHL,
C11H12N4SO—3280s1644w2947w1499vs1375m1073m836s3045w[L2Zn] ⋅ H2O,
C22H24N8S2O3Zn3433m3230s1600w2996w1497m1383m1046m720m3073w[L2Cu], C22H22N8S2O2Cu—3228s1614w2927w1499s1388s1050m734m3084w[L2], C22H22N8S2O2Ni—3328s1613w2929w1501m1391s1042m723m[L2Co], C22H22N8S2O2Co—3230s1616w2992w1496m1379s1039w718m3072wAll these data suggest the presence
of the free MBOBT in the form A in the solid state. This band is red shifted by
ca. 18–44cm−1   upon complex formation supporting the bonding of
oxygen to the metal ion. Accordingly, BMMB acts as a monobasic bidentate ligand
coordinated to the metal ions through the deprotonated thiolo-sulphur and
keto-oxygen atoms [18, 19].


### 3.2.2. Electronic Spectra and Magnetic
Studies

The
electronic spectra of the complexes, Table3, show intense bands at 26700–28600
and 29300–29700 cm−1 attributable to the intra-ligand O–M(II)
transitions suggesting the bonding of the ligand oxygen to the metal ion. The
spectra also exhibit a strong band at 22300–24700 cm−1 characteristic of S–M(II) LMCT transition and further support the bonding of
the ligand to the metal ion via a
sulphur atom.Table 3
Electronic spectral data (cm−1) for MBOBT complexes.
CompoundIntraligand
and CT transitionsd-d
transitions[L2Zn] ⋅ H2O,
C22H24N8S2O3Zn29280,
26850, 23700—[L2Cu], C22H22N8S2O2Cu26200—[L2Ni], C22H22N8S2O2Ni29700,
28600, 2450024200,19950, 16890[L2Co], C22H22N8S2O2Co29620,
26700, 247008500, 19960The electronic
spectrum of [L2Zn] ⋅ H2O shows intense bands at 29280, 26850, and 23700 cm−1
which
are assigned to the intraligands O → Zn(II) and S
→ Zn(II) LMCT, respectively. These
spectral features indicate the bonding of BMMB to the Zn(II) via oxygen and
sulphur atoms. The spectrum shows no bands in the region below 23000 cm−1 which is in accordance with the d10 electronic configuration of
Zn(II).Copper(II)
complex [L2Cu] gives a room temperature magnetic moment value of
1.78 B.M. characteristic of magnetically diluted copper(II) species. Its
electronic spectrum displays only an intense band at 26200 cm−1 which
is attributed to the intraligand (ligand localized) and LMCT transitions and
characteristic of a tetragonally distorted copper(II) complexes.The
electronic spectrum of [L2Ni] displays bands at, 16890, 19950, and
24200 cm−1  assignable to  1A1gυ→1A2g, (υ1), 1A1g→1B1g(υ2),   and 1A1g→1Eg(υ3) transitions, respectively, characteristic of
square planar nickel(II) complexes. The first two bands are pure d-d
transitions while theυ3 band obviously enveloped by a strong
CT transition. The assumed square planar geometry for this complex is confirmed
from the value of its room temperature magnetic moment of zero.The room
temperature magnetic moment of [L2Co] of 2.70 B.M. is more than that
of low spin octahedral and lower than the values characteristic of tetrahedral
cobalt(II) complexes. Furthermore, these values are similar to that reported
for the square planar cobalt(II) complexes [20, 21]. The electronic spectrum of
the complex exhibits two bands at 8500 and 19960 cm−1 characteristic
of square planar cobalt(II) complexes with a transition involving nonbonding
rather antibonding orbitals. In a strong field, the ground state is probably  2A1g with the configuration of eg4b2g2a1g1.


### 3.2.3. Thermal Analysis

The thermogravimetric (TG) and the
derivative thermogravimetric (DTG) plots of the complexes in the 25–1200∘C
range under N2  are shown in Figures 1–4. Their stepwise thermal degradation data are
given in Table 4. All complexes show two-stage mass loss
except [L2Zn]⋅H2O shows three decomposition steps.Table 4
Stepwise
thermal degradation data obtained from TGA curves for the metal complexes.
ComplexMolar massTG range
(C∘)DTGmax (C∘)Weight lossPredicated
intermediates and final productsMetallic residue (calcd. %) foundCalcd.Found[L2Zn] ⋅ H2O,
C22H24N8S2O3Zn577.332–120723.12.9H2OZnS136–32826149.849.22-BTA(16.5) 17.4[L2Cu], C22H22N8S2O2Cu557.5158–31621651.651.22-BTACuS449–54950031.230.9SO2  +  L1(17.2) 17.9[L2Ni],
C22H22N8S2O2Ni552.732–99781.61.51/2 H2O222–33129052.151.82-BTANiS414–60853131.431.1SO2 + L1(16.4) 17.1[L2Co],
C22H22N8S2O2Co552.9222–33129052.0852.72-BTACoS414–60853131.429.9SO2 + L1(16.5) 17.4222–33129052.0852.72-BTACoSBTA  =  benzotriazole ring, L1=  C6H10N2.Figure 1
TG and DTG plots of [L2Zn]⋅H2O.Scheme 1Figure 2
TG and DTG plots of
[L2Cu].Figure 3
TG and DTG plots of [L2Ni].Figure 4
TG and DTG plots of [L2Co].The
TG and DTG curves of [L2Zn]⋅H2O are shown in Figure 1. The
TGA curve of this complex shows three stages of decomposition within the
temperature range (32–654∘C). The first step of decomposition within the
temperature range (32–120∘C) corresponds to
the loss of water molecule of hydration with mass loss of 2.9% (calcd. 3.1%). The
second step (136–328∘C)
corresponds to the loss of two benzotriazole (BTA) moities and two acetylene
molecules (mass loss 49.2%; calcd. 49.8%). The third step (545–654∘C)
corresponds to the loss of SO2 and  L1 molecules (mass loss
30.1%; calcd. 29.8%). The energies of activation were 43.73, 37.1 and 24.4 kJ mol−1 for the first, second, and third steps, respectively. The
total mass loss up to 654∘C is in agreement with the formation
of ZnS as the final residue (TG 16.1%, calcd. 16.4%).The
thermogram given in Figure2 of [L2Cu] exhibits two significant
thermal events within the temperature range (158–549∘C).
The first step of decomposition within the
temperature range (158–316) corresponds to the loss of two BTA and two
acetylene molecules with a mass loss 51.2% (calcd. 51.6%). The second step
(449–549∘C) corresponds to the loss of
SO2 and  L1  molecules (mass loss 30.9%; calcd. 31.2%). The energies of
activation were 73.83 and 26.36 kJ mol−1 for the first and second
steps, respectively. The total mass loss up to 549∘C is in agreement
with the formation of CuS as the final residue (TG 17.2%, calcd. 17.9%).The
TG and DTG curves of [L2Ni] are shown in Figure 3. The TGA curve shows
two stages of decomposition within the temperature range (222–608∘C). The first step of decomposition within the
temperature range (222–331∘C) corresponds to the loss of
two
BTA and two acetylene molecules with a mass
loss of 52.1% (calcd. 51.8%). The second step (414–608∘C) corresponds to
the loss of L1  molecule (mass loss 31.4%; calcd. 31.1%). The
energies of activation were 64.3 and 16.7 kJ mol−1 for the first and
second steps, respectively. The total mass loss up to 608∘C is in agreement
with the formation of NiS as the final residue (TG 16.4%, calcd. 17.1%).The
[L2Co] complex is thermally stable up to 670∘C; see
Figure 4.
From the TG curve it appears that the complex decomposes in two stages over the
temperature range 160–670∘C. The first
decomposition occurs between 160–237∘C with mass loss of (calcd.
51.6%) and the second decomposition starts at 237∘C and ends at 670∘C with a 31.1% mass
loss (calcd. 31.4%). The first step of decomposition corresponds to the loss of
two BTA and two acetylene molecules
while  the second step corresponds to the loss of SO2 and   L1
molecules. The energies  of activation were 59.5 and 14.0 kJ mol−1  for the first and second steps, respectively.


### 3.2.4. Kinetic Data for the
Decomposition of Complexes

The
thermodynamic parameters of
decomposition processes of complexes, namely, activation energy (Ea),
enthalpy (ΔH*),
entropy (ΔS*),
and Gibbs free energy change of (ΔG*)
were evaluated graphically by employing the Coats-Redfern  method [22, 23]. This method, reviewed by Johnson and Gallagher [23] as
an integral method assuming various orders of reaction and comparing the
linearity in each case to select the correct order by using(1)log⁡[1−(1−α)1−nT2(1−n)]=log⁡[ARθEa(1−2RTEa)]−Ea2.303RTforn≠1,log⁡{−log⁡(1−α)T2}=log⁡[ARθEa(1−2RTEa)]−Ea2.303RTforn=1, where α is the
fraction of sample decomposed at time t, T  is the derivative peak temperature,
A is the frequency factor, Ea is the activation energy, R is the gas constant, θ is the heating rate, and  (1 − (2RT/Ea)) ≅ 1. A plot of log{− log (1−a)/T2} versus 1/T
gives a slope from which the Ea was calculated and A (Arrhenius
factor) was determined from the intercept. Trials of these plots were made by
assuming the orders 0, 1/2, and 1 and the best plot was obtained for the first
order. The entropy of activation was calculated using [24](2)ΔS*=2.303R[log⁡(AhkT)], where h and k  stand for the Planck and
Boltzmann constants, respectively, and T is the peak temperature from the DTG
curve. The free energy of activation  ΔG* and
the enthalpy of activation  ΔH* are
calculated using (4), (3)ΔH*=Ea−RT,ΔG*=ΔH*−TΔS*. The kinetic data obtained from the
nonisothermal decomposition of the complexes are given in Table 5.Table 5
The kinetic parameters for the nonisothermal
decomposition of the complexes.
Complex/range (C∘)T(a)Ea (KJ mol−1)A (S−1)ΔH*
(kJ mol−1)ΔS*
(JK−1mol−1)ΔG*
(kJ mol−1)[L2Cu
]90–13421673.831.57E + 0469.76−169.14152.47220–26050026.364.0E− 0319.93−299.09251.13[L2Ni]222–33929064.353.7E + 0269.70−201.45173.08430–60853116.711.5E− 0310.03−307.60257.30[L2Co]160–32722759.532.29E + 0255.37−204.40157.60327–67045214.042.67E− 048.01−321.10240.80[L2Zn] ⋅ H2O32–1207243.731.91E + 0340.87−183.70104.06153–33626137.084.15E− 0132.64−257.50170.12544–66259524.401.29E− 0317.23−309.50285.87(a)The peak
temperature from the DTG curve.The activation energy of the
complexes is expected to increase with decreasing metal ion radius [25, 26].
The smaller size of metal ions permits a closer approach of the ligand.Hence, theΔE*
value in
the first stages for the Cu(II) complex is higher than those of Ni(II), Co(II),
and Zn(II) complexes [27–29]. The calculated ΔE*
values using
Coats-Redfern method for the first-stage decomposition of the complexes are
found to beΔECu*=  73.8 kJ mol−1 > ΔENi* =  64.3 kJ mol−1 >  ΔECo*  =  59.5 kJ mol−1 which is in accordance with rCu(II)= 70 pm  <  rNi(II) =  72 pm  <  rCo(II) =  74 pm.The same decomposition kinetics is
also true for theΔE* values of the second stage decomposition which
was found to be in the following order:ΔECu* =  26.3 kJ mol−1> ΔENi* =  16.7 kJ mol−1 >  ΔECo* = 14.0 kJ mol−1.The negative values ofΔS*, see Table 5, indicate that the reaction rates are slower than normal [30] which is
consistent with the results reported previously [31]. Furthermore, these data
indicate that the activated complexes have more ordered structure than the
reactants [29–31].


### 3.2.5. Biological Activity

The antibacterial activity of MBOBT
and its metal(II) complexes are given inTables 6 and 7 and the average of three experimental data for [L2Zn]⋅H2O
and [L2Cu] are shown in Figures 5–15. The results show that (i) the complexes exhibit inhibitory effects
towards the activity of gram-positive and gram-negative bacteria in contrast to
the parent organic ligand which is biologically inactive under the experimental
conditions, (ii) all complexes are inactive towards Salmonellasp2 and only [L2Cu] is active towards
Staphylococcus aurous, and (iii) copper(II) complex has a wide spectrum with respect
to the studied bacteria. As previously reported, the metal salts do not exhibit
antimicrobial activity [32–36]. The biological activity of the metal complexes
is governed by the following factors [36]: (i) the chelate effect of the ligands,
(ii) the nature of the donor atoms, (iii) the total charge on the complex ion, (iv)
the nature of the metal ion, (v) the nature of the counter ions that neutralize
the complex, and (vi) the geometrical structure of the complex [35].
Furthermore, chelation reduces the polarity of the metal ion because of partial
sharing of its positive charge with the donor groups and possibly the π-electron
delocalization within the whole chelate ring system that is formed during
coordination [33]. These factors increase the lipophilic nature of the central
metal atom and hence increasing the hydrophobic character and liposolubility of
the molecule favoring its permeation through the lipid bilayer of the bacterial
membrane. This enhances the rate of uptake/entrance and thus the antibacterial
activity of the testing compounds. Accordingly, the antimicrobial activity of
the four complexes can be referred to the increase of their lipophilic
character which in turn deactivates enzymes responsible for respiratory
processes and probably other cellular enzymes, which play a vital role in
various metabolic pathways of the tested bacteria. Also it is proposed that the
action of the toxicant is the denaturation of one or more proteins of the cell
and this impairs normal cellular process. According to the data given in Tables
6 and 7, the antimicrobial activity can be ordered as [L2Cu]
> [L2Zn]⋅H2O >
[L2Ni] > [L2Co],
suggesting that the lipophilic behavior
increases in the same order. Since all complexes (i) have the same
donating atoms which are S/O with the same coordination number (C.N. for each
is 4), (ii) have the same chelate effect (all form two 6-membered chelating
rings), (iii) are neutral and there are no counter ions, and (iv) have the same
oxidation number in their complexes (M2+), therefore, the more
effective factors are the geometrical shape and the nature of the central
atoms. According to the spectral and magnetic studies, (i) copper has a
tetragonal distortion (distorted to a tetrahedral geometry); (ii) cobalt and
nickel have a square planar; (iii) zinc is associated with a tetrahedral
geometry. Therefore, the higher antimicrobial activity can be referred to their
similar structure which is the tetrahedral. This structure increases the
lipophilicity of the central atom by decreasing the effective nuclear charge
(polarity) of the Zn(II) and Cu(II) more than the square planar structure of
Co(II) and Ni(II). The higher antimicrobial activity of copper(II) complex
relative to the zinc(II) complex may be referred to the presence of water
molecule in the formula of the later complex, also copper(II) may form stronger
copper(II)-ligand bond than Zn(II)-ligand bond and this in turn increases the lipophilic
character of copper(II) complex than zinc(II) complex. The redox activity of
copper compared to zinc, which is redox neutral, may be taken as an additional
reason for the higher activity of copper relative to zinc complex.Table 6
Effect of MBOBT and its complexes on the
respiration of bacteria. (Results represent percent
inhibition of bacterial respiration caused by the addition of 0.5 and 1.0 mg
of the test compound. Results are the mean of
three independent analyses with standard
deviations.)
CompoundBacteriaStaphylococcus aureusStaphylococcus hominisBacillussp1Bacillussp2Bacillussp3Escherichia coliSalmonellasp1Salmonellasp2[L2Zn] ⋅ H2OAmount (mg)0.5nilnil94.7±6.353.2±3.584.3±5.6nil40.7±2.5nil1.0nilnil93.1±6.394.5±6.386.1±5.7nil79.1±5.3nil[L2Cu]0.589.6±6.087.3±5.884.6±5.690.3±6.088.5±5.892.6±6.267.7±4.5nil1.094.5±6.391.0±6.095.1±6.566.7±4.487.9±6.093.7±6.474.8±5.0nil[L2Ni]0.5nilnil68.3±4.673.5±4.944.6±3.0nilnilnil1.0nil16.5±1.089.5±6.090.8±6.067.2±4.5nilnilnil[L2Co]0.5nilnil26.8±1.736.5±2.433.2±2.2nilnilnil1.0nilnil69.9±4.778.9±5.370.1±4.6nilnilnilBMMB0.5nilnilnilnilnilnilnilnil1.0nilnilnilnilnilnilnilnilTable 7
Antimicrobial results (zone of inhibition, diameter in cm) of MBOBT and its
complexes using gel-diffusion
method.
CompoundBacteriaStaphylococcus aureusStaphylococcus hominisBacillussp1Bacillussp2Bacillussp3Escherichia coliSalmonellasp1Salmonellasp2[L2Z] ⋅ H2Onilnil3 ±  0.11.8±0.051.5±0.05nil1.2±0.04nil[L2Cu]1 ±  0.031.2±0.043.2±0.12  ± 0.062.1±0.061.2±0.041.9±0.06nil[L2Ni]1.2±0.041 ±  0.030.9±0.030.9±0.031 ±  0.04nil1.4±0.05nil[L2Co]0.9±0.031 ±  0.031.2±0.041.2±0.040.9±0.030.9±0.03nilnilBMMBnilnilnilnilnilnilnilnilFigure 5
Effect of
[L2Cu] on the respiration of  Bacillussp1.Figure 6
Effect
of [L2Cu] on the respiration of Bacillussp2.Figure 7
Effect
of [L2Cu] on the respiration of Bacillussp3.Figure 8
Effect
of [L2Cu] on the respiration of
E. coli.Figure 9
Effect
of [L2Cu] on the respiration of Salmonellasp1.Figure 10
Effect
of [L2Cu] on the respiration of Staphylococcus aurous.Figure 11
Effect of [L2Cu]
on the respiration of Staphylococcus hominis.Figure 12
Effect
of [L2Zn]⋅H2O on the respiration of Salmonellasp1.Figure 13
Effect
of [L2Zn]⋅H2O on the respiration of Bacillussp1.Figure 14
Effect
of [L2Zn]⋅H2O on the respiration of Bacillussp2.Figure 15
Effect
of [L2Zn]⋅H2O on the respiration of Bacillussp3.


## 3.2.1. NMR and IR Spectra of MBOBT and Its Complexes

The 13C NMR spectrum ofMBOBT in d6-DMSO was recorded. Despite expecting signals for only
10 carbons, twenty carbon signals appeared with the spectra indicating that at
least in DMSO, the molecule exists as an equilibrium mixture of two forms (I A and
I B). The existence of a signal at δ 76.16 ppm characteristic of an sp3 carbon indicated clearly that one of these two forms is A. In the C=O region
two carbonyl carbons at δ 197.9 and 192.3 ppm are detected indicating the presence
of C=O in both forms. The spectrum
exhibits only one C=S signal at δ 197.98 ppm (should
be appeared at δ 173 ppm). Furthermore, the spectrum displays a signal at δ
173.4 ppm characteristic of an sp2 carbon in accordance with the
assumption of the second form B.The infrared spectra of MBOBT
and its different complexes are recorded as KBr discs and main bands with their
tentative assignments given in Table2. The spectrum of MBOBT does not show ν(SH) band at 2600–2500 cm−1, in which this stretching frequency is generally
expected and is therefore mainly in the
thioketo form [15]. The spectrum of MBOBT displays four bands
1499,
1375, 1073, and 836 cm−1 assigned to the thioamide bands, namely I, II, III, and IV, contains a
thioamide group (HNC=S), and has contribution from δ(C–H) + δ(N–H), ν(C=S)  +  ν(C=N)  +  ν(C–H), ν(C–N)  +  ν(C–S),
and ν(C…S),
respectively [16, 17]. These thioamide bands III and
IV are strongly shifted to lower wavenumbers in the spectra of all complexes
supporting sulphur donation and deprotonation of the ligand as well.
Furthermore, the thioamide bands I and II are not greatly affected by
complexation suggesting the nonbonding nature of the nitrogen to the metal ion.
The spectrum of MBOBT displays only a weak C=O absorption at 1644 cm−1 and medium-strong band at 3280 cm−1 due to υ(NH). Shifting position and
decreasing intensity of the υ(C=O) and shifting of the
υ(NH) to longer
wavelength and  increasing intensity may
be due to hydrogen bonding formation between these two groups. As expected, CH
stretches for C–H linked to sp2 and sp3 carbon  appeared
at 3045 and 2947 cm−1, respectively.Table 2
Main
IR (ύ, cm−1)
bands for MBOBT
and its metal(II) complexes.  (w  =  week, m  =  medium, s = strong.)
Compoundυ(OH)υ(NH)υ(C=O)υ(CH3)υ(thioamide)IIIIIIIVHL,
C11H12N4SO—3280s1644w2947w1499vs1375m1073m836s3045w[L2Zn] ⋅ H2O,
C22H24N8S2O3Zn3433m3230s1600w2996w1497m1383m1046m720m3073w[L2Cu], C22H22N8S2O2Cu—3228s1614w2927w1499s1388s1050m734m3084w[L2], C22H22N8S2O2Ni—3328s1613w2929w1501m1391s1042m723m[L2Co], C22H22N8S2O2Co—3230s1616w2992w1496m1379s1039w718m3072wAll these data suggest the presence
of the free MBOBT in the form A in the solid state. This band is red shifted by
ca. 18–44cm−1   upon complex formation supporting the bonding of
oxygen to the metal ion. Accordingly, BMMB acts as a monobasic bidentate ligand
coordinated to the metal ions through the deprotonated thiolo-sulphur and
keto-oxygen atoms [18, 19].


## 3.2.2. Electronic Spectra and Magnetic
Studies

The
electronic spectra of the complexes, Table3, show intense bands at 26700–28600
and 29300–29700 cm−1 attributable to the intra-ligand O–M(II)
transitions suggesting the bonding of the ligand oxygen to the metal ion. The
spectra also exhibit a strong band at 22300–24700 cm−1 characteristic of S–M(II) LMCT transition and further support the bonding of
the ligand to the metal ion via a
sulphur atom.Table 3
Electronic spectral data (cm−1) for MBOBT complexes.
CompoundIntraligand
and CT transitionsd-d
transitions[L2Zn] ⋅ H2O,
C22H24N8S2O3Zn29280,
26850, 23700—[L2Cu], C22H22N8S2O2Cu26200—[L2Ni], C22H22N8S2O2Ni29700,
28600, 2450024200,19950, 16890[L2Co], C22H22N8S2O2Co29620,
26700, 247008500, 19960The electronic
spectrum of [L2Zn] ⋅ H2O shows intense bands at 29280, 26850, and 23700 cm−1
which
are assigned to the intraligands O → Zn(II) and S
→ Zn(II) LMCT, respectively. These
spectral features indicate the bonding of BMMB to the Zn(II) via oxygen and
sulphur atoms. The spectrum shows no bands in the region below 23000 cm−1 which is in accordance with the d10 electronic configuration of
Zn(II).Copper(II)
complex [L2Cu] gives a room temperature magnetic moment value of
1.78 B.M. characteristic of magnetically diluted copper(II) species. Its
electronic spectrum displays only an intense band at 26200 cm−1 which
is attributed to the intraligand (ligand localized) and LMCT transitions and
characteristic of a tetragonally distorted copper(II) complexes.The
electronic spectrum of [L2Ni] displays bands at, 16890, 19950, and
24200 cm−1  assignable to  1A1gυ→1A2g, (υ1), 1A1g→1B1g(υ2),   and 1A1g→1Eg(υ3) transitions, respectively, characteristic of
square planar nickel(II) complexes. The first two bands are pure d-d
transitions while theυ3 band obviously enveloped by a strong
CT transition. The assumed square planar geometry for this complex is confirmed
from the value of its room temperature magnetic moment of zero.The room
temperature magnetic moment of [L2Co] of 2.70 B.M. is more than that
of low spin octahedral and lower than the values characteristic of tetrahedral
cobalt(II) complexes. Furthermore, these values are similar to that reported
for the square planar cobalt(II) complexes [20, 21]. The electronic spectrum of
the complex exhibits two bands at 8500 and 19960 cm−1 characteristic
of square planar cobalt(II) complexes with a transition involving nonbonding
rather antibonding orbitals. In a strong field, the ground state is probably  2A1g with the configuration of eg4b2g2a1g1.


## 3.2.3. Thermal Analysis

The thermogravimetric (TG) and the
derivative thermogravimetric (DTG) plots of the complexes in the 25–1200∘C
range under N2  are shown in Figures 1–4. Their stepwise thermal degradation data are
given in Table 4. All complexes show two-stage mass loss
except [L2Zn]⋅H2O shows three decomposition steps.Table 4
Stepwise
thermal degradation data obtained from TGA curves for the metal complexes.
ComplexMolar massTG range
(C∘)DTGmax (C∘)Weight lossPredicated
intermediates and final productsMetallic residue (calcd. %) foundCalcd.Found[L2Zn] ⋅ H2O,
C22H24N8S2O3Zn577.332–120723.12.9H2OZnS136–32826149.849.22-BTA(16.5) 17.4[L2Cu], C22H22N8S2O2Cu557.5158–31621651.651.22-BTACuS449–54950031.230.9SO2  +  L1(17.2) 17.9[L2Ni],
C22H22N8S2O2Ni552.732–99781.61.51/2 H2O222–33129052.151.82-BTANiS414–60853131.431.1SO2 + L1(16.4) 17.1[L2Co],
C22H22N8S2O2Co552.9222–33129052.0852.72-BTACoS414–60853131.429.9SO2 + L1(16.5) 17.4222–33129052.0852.72-BTACoSBTA  =  benzotriazole ring, L1=  C6H10N2.Figure 1
TG and DTG plots of [L2Zn]⋅H2O.Scheme 1Figure 2
TG and DTG plots of
[L2Cu].Figure 3
TG and DTG plots of [L2Ni].Figure 4
TG and DTG plots of [L2Co].The
TG and DTG curves of [L2Zn]⋅H2O are shown in Figure 1. The
TGA curve of this complex shows three stages of decomposition within the
temperature range (32–654∘C). The first step of decomposition within the
temperature range (32–120∘C) corresponds to
the loss of water molecule of hydration with mass loss of 2.9% (calcd. 3.1%). The
second step (136–328∘C)
corresponds to the loss of two benzotriazole (BTA) moities and two acetylene
molecules (mass loss 49.2%; calcd. 49.8%). The third step (545–654∘C)
corresponds to the loss of SO2 and  L1 molecules (mass loss
30.1%; calcd. 29.8%). The energies of activation were 43.73, 37.1 and 24.4 kJ mol−1 for the first, second, and third steps, respectively. The
total mass loss up to 654∘C is in agreement with the formation
of ZnS as the final residue (TG 16.1%, calcd. 16.4%).The
thermogram given in Figure2 of [L2Cu] exhibits two significant
thermal events within the temperature range (158–549∘C).
The first step of decomposition within the
temperature range (158–316) corresponds to the loss of two BTA and two
acetylene molecules with a mass loss 51.2% (calcd. 51.6%). The second step
(449–549∘C) corresponds to the loss of
SO2 and  L1  molecules (mass loss 30.9%; calcd. 31.2%). The energies of
activation were 73.83 and 26.36 kJ mol−1 for the first and second
steps, respectively. The total mass loss up to 549∘C is in agreement
with the formation of CuS as the final residue (TG 17.2%, calcd. 17.9%).The
TG and DTG curves of [L2Ni] are shown in Figure 3. The TGA curve shows
two stages of decomposition within the temperature range (222–608∘C). The first step of decomposition within the
temperature range (222–331∘C) corresponds to the loss of
two
BTA and two acetylene molecules with a mass
loss of 52.1% (calcd. 51.8%). The second step (414–608∘C) corresponds to
the loss of L1  molecule (mass loss 31.4%; calcd. 31.1%). The
energies of activation were 64.3 and 16.7 kJ mol−1 for the first and
second steps, respectively. The total mass loss up to 608∘C is in agreement
with the formation of NiS as the final residue (TG 16.4%, calcd. 17.1%).The
[L2Co] complex is thermally stable up to 670∘C; see
Figure 4.
From the TG curve it appears that the complex decomposes in two stages over the
temperature range 160–670∘C. The first
decomposition occurs between 160–237∘C with mass loss of (calcd.
51.6%) and the second decomposition starts at 237∘C and ends at 670∘C with a 31.1% mass
loss (calcd. 31.4%). The first step of decomposition corresponds to the loss of
two BTA and two acetylene molecules
while  the second step corresponds to the loss of SO2 and   L1
molecules. The energies  of activation were 59.5 and 14.0 kJ mol−1  for the first and second steps, respectively.


## 3.2.4. Kinetic Data for the
Decomposition of Complexes

The
thermodynamic parameters of
decomposition processes of complexes, namely, activation energy (Ea),
enthalpy (ΔH*),
entropy (ΔS*),
and Gibbs free energy change of (ΔG*)
were evaluated graphically by employing the Coats-Redfern  method [22, 23]. This method, reviewed by Johnson and Gallagher [23] as
an integral method assuming various orders of reaction and comparing the
linearity in each case to select the correct order by using(1)log⁡[1−(1−α)1−nT2(1−n)]=log⁡[ARθEa(1−2RTEa)]−Ea2.303RTforn≠1,log⁡{−log⁡(1−α)T2}=log⁡[ARθEa(1−2RTEa)]−Ea2.303RTforn=1, where α is the
fraction of sample decomposed at time t, T  is the derivative peak temperature,
A is the frequency factor, Ea is the activation energy, R is the gas constant, θ is the heating rate, and  (1 − (2RT/Ea)) ≅ 1. A plot of log{− log (1−a)/T2} versus 1/T
gives a slope from which the Ea was calculated and A (Arrhenius
factor) was determined from the intercept. Trials of these plots were made by
assuming the orders 0, 1/2, and 1 and the best plot was obtained for the first
order. The entropy of activation was calculated using [24](2)ΔS*=2.303R[log⁡(AhkT)], where h and k  stand for the Planck and
Boltzmann constants, respectively, and T is the peak temperature from the DTG
curve. The free energy of activation  ΔG* and
the enthalpy of activation  ΔH* are
calculated using (4), (3)ΔH*=Ea−RT,ΔG*=ΔH*−TΔS*. The kinetic data obtained from the
nonisothermal decomposition of the complexes are given in Table 5.Table 5
The kinetic parameters for the nonisothermal
decomposition of the complexes.
Complex/range (C∘)T(a)Ea (KJ mol−1)A (S−1)ΔH*
(kJ mol−1)ΔS*
(JK−1mol−1)ΔG*
(kJ mol−1)[L2Cu
]90–13421673.831.57E + 0469.76−169.14152.47220–26050026.364.0E− 0319.93−299.09251.13[L2Ni]222–33929064.353.7E + 0269.70−201.45173.08430–60853116.711.5E− 0310.03−307.60257.30[L2Co]160–32722759.532.29E + 0255.37−204.40157.60327–67045214.042.67E− 048.01−321.10240.80[L2Zn] ⋅ H2O32–1207243.731.91E + 0340.87−183.70104.06153–33626137.084.15E− 0132.64−257.50170.12544–66259524.401.29E− 0317.23−309.50285.87(a)The peak
temperature from the DTG curve.The activation energy of the
complexes is expected to increase with decreasing metal ion radius [25, 26].
The smaller size of metal ions permits a closer approach of the ligand.Hence, theΔE*
value in
the first stages for the Cu(II) complex is higher than those of Ni(II), Co(II),
and Zn(II) complexes [27–29]. The calculated ΔE*
values using
Coats-Redfern method for the first-stage decomposition of the complexes are
found to beΔECu*=  73.8 kJ mol−1 > ΔENi* =  64.3 kJ mol−1 >  ΔECo*  =  59.5 kJ mol−1 which is in accordance with rCu(II)= 70 pm  <  rNi(II) =  72 pm  <  rCo(II) =  74 pm.The same decomposition kinetics is
also true for theΔE* values of the second stage decomposition which
was found to be in the following order:ΔECu* =  26.3 kJ mol−1> ΔENi* =  16.7 kJ mol−1 >  ΔECo* = 14.0 kJ mol−1.The negative values ofΔS*, see Table 5, indicate that the reaction rates are slower than normal [30] which is
consistent with the results reported previously [31]. Furthermore, these data
indicate that the activated complexes have more ordered structure than the
reactants [29–31].


## 3.2.5. Biological Activity

The antibacterial activity of MBOBT
and its metal(II) complexes are given inTables 6 and 7 and the average of three experimental data for [L2Zn]⋅H2O
and [L2Cu] are shown in Figures 5–15. The results show that (i) the complexes exhibit inhibitory effects
towards the activity of gram-positive and gram-negative bacteria in contrast to
the parent organic ligand which is biologically inactive under the experimental
conditions, (ii) all complexes are inactive towards Salmonellasp2 and only [L2Cu] is active towards
Staphylococcus aurous, and (iii) copper(II) complex has a wide spectrum with respect
to the studied bacteria. As previously reported, the metal salts do not exhibit
antimicrobial activity [32–36]. The biological activity of the metal complexes
is governed by the following factors [36]: (i) the chelate effect of the ligands,
(ii) the nature of the donor atoms, (iii) the total charge on the complex ion, (iv)
the nature of the metal ion, (v) the nature of the counter ions that neutralize
the complex, and (vi) the geometrical structure of the complex [35].
Furthermore, chelation reduces the polarity of the metal ion because of partial
sharing of its positive charge with the donor groups and possibly the π-electron
delocalization within the whole chelate ring system that is formed during
coordination [33]. These factors increase the lipophilic nature of the central
metal atom and hence increasing the hydrophobic character and liposolubility of
the molecule favoring its permeation through the lipid bilayer of the bacterial
membrane. This enhances the rate of uptake/entrance and thus the antibacterial
activity of the testing compounds. Accordingly, the antimicrobial activity of
the four complexes can be referred to the increase of their lipophilic
character which in turn deactivates enzymes responsible for respiratory
processes and probably other cellular enzymes, which play a vital role in
various metabolic pathways of the tested bacteria. Also it is proposed that the
action of the toxicant is the denaturation of one or more proteins of the cell
and this impairs normal cellular process. According to the data given in Tables
6 and 7, the antimicrobial activity can be ordered as [L2Cu]
> [L2Zn]⋅H2O >
[L2Ni] > [L2Co],
suggesting that the lipophilic behavior
increases in the same order. Since all complexes (i) have the same
donating atoms which are S/O with the same coordination number (C.N. for each
is 4), (ii) have the same chelate effect (all form two 6-membered chelating
rings), (iii) are neutral and there are no counter ions, and (iv) have the same
oxidation number in their complexes (M2+), therefore, the more
effective factors are the geometrical shape and the nature of the central
atoms. According to the spectral and magnetic studies, (i) copper has a
tetragonal distortion (distorted to a tetrahedral geometry); (ii) cobalt and
nickel have a square planar; (iii) zinc is associated with a tetrahedral
geometry. Therefore, the higher antimicrobial activity can be referred to their
similar structure which is the tetrahedral. This structure increases the
lipophilicity of the central atom by decreasing the effective nuclear charge
(polarity) of the Zn(II) and Cu(II) more than the square planar structure of
Co(II) and Ni(II). The higher antimicrobial activity of copper(II) complex
relative to the zinc(II) complex may be referred to the presence of water
molecule in the formula of the later complex, also copper(II) may form stronger
copper(II)-ligand bond than Zn(II)-ligand bond and this in turn increases the lipophilic
character of copper(II) complex than zinc(II) complex. The redox activity of
copper compared to zinc, which is redox neutral, may be taken as an additional
reason for the higher activity of copper relative to zinc complex.Table 6
Effect of MBOBT and its complexes on the
respiration of bacteria. (Results represent percent
inhibition of bacterial respiration caused by the addition of 0.5 and 1.0 mg
of the test compound. Results are the mean of
three independent analyses with standard
deviations.)
CompoundBacteriaStaphylococcus aureusStaphylococcus hominisBacillussp1Bacillussp2Bacillussp3Escherichia coliSalmonellasp1Salmonellasp2[L2Zn] ⋅ H2OAmount (mg)0.5nilnil94.7±6.353.2±3.584.3±5.6nil40.7±2.5nil1.0nilnil93.1±6.394.5±6.386.1±5.7nil79.1±5.3nil[L2Cu]0.589.6±6.087.3±5.884.6±5.690.3±6.088.5±5.892.6±6.267.7±4.5nil1.094.5±6.391.0±6.095.1±6.566.7±4.487.9±6.093.7±6.474.8±5.0nil[L2Ni]0.5nilnil68.3±4.673.5±4.944.6±3.0nilnilnil1.0nil16.5±1.089.5±6.090.8±6.067.2±4.5nilnilnil[L2Co]0.5nilnil26.8±1.736.5±2.433.2±2.2nilnilnil1.0nilnil69.9±4.778.9±5.370.1±4.6nilnilnilBMMB0.5nilnilnilnilnilnilnilnil1.0nilnilnilnilnilnilnilnilTable 7
Antimicrobial results (zone of inhibition, diameter in cm) of MBOBT and its
complexes using gel-diffusion
method.
CompoundBacteriaStaphylococcus aureusStaphylococcus hominisBacillussp1Bacillussp2Bacillussp3Escherichia coliSalmonellasp1Salmonellasp2[L2Z] ⋅ H2Onilnil3 ±  0.11.8±0.051.5±0.05nil1.2±0.04nil[L2Cu]1 ±  0.031.2±0.043.2±0.12  ± 0.062.1±0.061.2±0.041.9±0.06nil[L2Ni]1.2±0.041 ±  0.030.9±0.030.9±0.031 ±  0.04nil1.4±0.05nil[L2Co]0.9±0.031 ±  0.031.2±0.041.2±0.040.9±0.030.9±0.03nilnilBMMBnilnilnilnilnilnilnilnilFigure 5
Effect of
[L2Cu] on the respiration of  Bacillussp1.Figure 6
Effect
of [L2Cu] on the respiration of Bacillussp2.Figure 7
Effect
of [L2Cu] on the respiration of Bacillussp3.Figure 8
Effect
of [L2Cu] on the respiration of
E. coli.Figure 9
Effect
of [L2Cu] on the respiration of Salmonellasp1.Figure 10
Effect
of [L2Cu] on the respiration of Staphylococcus aurous.Figure 11
Effect of [L2Cu]
on the respiration of Staphylococcus hominis.Figure 12
Effect
of [L2Zn]⋅H2O on the respiration of Salmonellasp1.Figure 13
Effect
of [L2Zn]⋅H2O on the respiration of Bacillussp1.Figure 14
Effect
of [L2Zn]⋅H2O on the respiration of Bacillussp2.Figure 15
Effect
of [L2Zn]⋅H2O on the respiration of Bacillussp3.


## 4. Summary and Conclusions

The interaction of the
newly synthesized MBOBT with Zn2+, Cu2+, Ni2+,
and Co2+ leads to the formation of neutral complexes [L2M]⋅nH2O.
Their structures and formation are determined using microanalysis, magnetic,
and different spectral tools. Copper and Zn(II) complexes are of a distorted
tetrahedral whereas cobalt(II) and
nickel(II) complexes are associated with square planar(II) structures. The
thermal analysis data showed that the stability of the complexes can be ordered
as [L2Cu]  > [L2Ni] > [L2Co]
> [L2Zn]⋅H2O. Furthermore, the negative values of ΔS*  indicate that the reaction rates are slower
than normal and the activated complexes have more ordered structure than the
reactants. The antimicrobial tests showed that (i) the complexes are
antimicrobial active while the free ligand BMMB is not, and (ii) the
copper(II) complex can be considered as
the most promising potent broad spectrum antimicrobial compound among the four
complexes, where it is found to be superior to all other complexes against all
the test organisms except Salmonellasp2.



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*Source: 479897-2008-02-28.xml*