In search of Benign methodologies for coordination: Synthesis, characterization, and in vitro biology of transition metal complexes of N, N′-bis (salicylidine) toluene-2, 4-diamine with binuclear core

Article abstract of DOI:10.1080/15533174.2010.509302

A series of transition metal complexes of Zn(II), Co(II), Cu(II), Ni(II), and Mn(II) have been synthesized from Schiff's base H2Sal-2,4-toluene, [N,N′-bis(salicylidine)toluene-2,4-diamine] (IV) derived from salicylaldehyde and toluene 2,4-diamine using MO

Full text of DOI:10.1080/15533174.2010.509302

Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 40:633–639, 2010
Copyright © Taylor & Francis Group, LLC
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2010.509302
In Search of Benign Methodologies for Coordination:
Synthesis, Characterization, and in vitro Biology of
Transition Metal Complexes of N, Nꢀ-bis (salicylidine)
toluene-2, 4-diamine with Binuclear Core
Parvez Ali,1 Jyotsna Meshram,1 Vandana Tiwari,1 Javed Sheikh,1
Sachin Wazalwar,1 Mudrika Ahemad,2 and Sunil Pandey3
1Department of Chemistry, Rashtrasant Tukadoji Maharaj Nagpur University, Nagpur, India
2G. H. Raisoni College of Engineering and Technology, Nagpur, India
3Rajiv Gandhi Biotechnology Centre, Rashtrasant Tukadoji Maharaj Nagpur University, Nagpur, India
Schiff bases continue to occupy an important position as ligands
in metal coordination chemistry even almost a century since
their discovery. Much work has been done[6–10] on the com-
plexation of Schiff’s base skeleton with transition metals, i.e.,
iron, cobalt, nickel, and copper. The synthesis of coordinating
agents produced from aromatic diamines is of special interest
since the use of nitrogen as a donor atom for coordination of a
single cation is directly related to their relative ortho, meta, or
para positions, as shown for tetra methyl carboxylic acids.[11−15]
The reactions of tetra dentate Schiff’s bases derived from
salicylaldehyde and diamine has been the subject of many
studies.[16−18] Diamines have received the considerable atten-
tion because of their potential as catalysts for insertion of oxy-
gen into an organic substrate.[19−20] However, these reactions
suffered from drawbacks such as longer reaction times, the use
of high boiling solvents, low yields, and the use of reactants with
high toxicity, which limit their use for the synthesis of complex
molecules.
A series of transition metal complexes of Zn(II), Co(II), Cu(II),
Ni(II), and Mn(II) have been synthesized from Schiff’s base
H2Sal-2,4-toluene, [N,Nꢀ-bis(salicylidine)toluene-2,4-diamine] (IV)
derived from salicylaldehyde and toluene 2,4-diamine using MORE
technique. Structural features were obtained from FTIR, UV-VIS,
molar conductance, ELEMENTAL analysis, 1H-NMR, mass spec-
tral and X-ray diffraction studies, which have shown the formation
of dimer complexes, where the ligand acts as a bridge between two
metal cations. Ligand and complexes have been screened experi-
mentally for their antimicrobial activity against different species
of pathogenic bacteria; Viz., E. coli, S. aureus, B. subtilis and P.
aureginosam and their antimicrobial potency have been discussed.
Keywords antimicrobials, coordination, ligand, metal complexes,
MORE, Schiff’s base
1. INTRODUCTION
Schiff’s bases and the relevant transition metal complexes are
widely studied because they have industrial, antifungal, antibac-
terial, anticancer, and herbicidal applications.[1] These com-
plexes play an important role in the development of coordination
chemistry[2] and possess important properties, such as biological
activity,[3] catalytic activity,[4] and photochromic properties.[5]
Mederos et al.[21] have reported similar types of complexes
by conventional approach that suffered from the above discussed
drawbacks. Thus, inorder toeliminate that typeof discrepancies,
a simple, cost effective, and efficient approach for the synthesis
of coordination complexes is demonstrated.
Reactions that proceed in a matter of hours or days when
heated by conventional means have been shown to go to
completion in several minutes when exposed to microwaves.[22]
Speeding up a chemical reaction to this degree is not only much
more energy efficient, but often also results in much cleaner pro-
cesses, i.e., the development of unwanted byproducts that may
form when a reaction is heated for extended periods of time are
reduced or eliminated completely. Microwave irradiation often
leads to shorter reaction times, increased yields, easier workup,
matches with green chemistry protocols, and can enhance the
selectivity of reactions. Furthermore, its unique capabilities
allow its application in reactions that are difficult or impossible
Received 7 March 2010; accepted 12 July 2010.
Financial support from University Grant Commission [F-33-
301/2007(SR)], New Delhi is highly acknowledged. The authors are
thankful to Rajiv Gandhi Biotechnology Centre, Rashtrasant Tukadoji
Maharaj Nagpur University, Nagpur for conducting biological screen-
ing. Authors are thankful to VNIT, Nagpur for providing FTIR spectral
analysis. The authors are grateful to IICT Hyderabad, for the help in
undertaking NMR and Mass spectra.
Address correspondence to Parvez Ali, Department of Chemistry,
Rashtrasant Tukadoji Maharaj Nagpur University, Nagpur-440033,
India. E-mail: parvezali 81@yahoo.com
633

634
P. ALI ET AL.
2. EXPERIMENTAL
2.1. Starting Materials
The solvents and reagents used in the synthetic work were of
laboratory grade and were purified by distillation or crystalliza-
tion where necessary, and their boiling or melting points were
compared with the available literature values. Melting points
were determined in open capillaries and are uncorrected. The
completion of reaction was monitored by thin-layer chromatog-
raphy (TLC) using silica gel-G coated plates (0.5 mm thickness,
Merck) and spots were visualized in UV chamber and by ex-
posing the dry plates to iodine vapors.
2.2. Physical Measurements
FTIR spectra were recorded using KBr pellets on a Shi-
madzu spectrophotometer in the range 400–4000 cm−1. Cintra
10e (UV visible) spectrophotometer was used to determine elec-
tronic spectra from 200–800 nm in DMSO. 1H NMR spectra of
Schiff’s base and metal complexes were recorded in DMSO-d6
using Varian XL-400 NMR spectrophotometer with TMS as an
internal standard. ESI mass spectra were recorded on a Varian
MAT CH-5 spectrometer. Elemental analyses were carried out
using a Perkin–Elmer, CHNS/O elemental analyzer model 2400.
XRD data were collected on PAN analytical Xpert-pro. Ultra-
violet visible fluorescence analysis cabinet (SONAR) was used
for visualization of TLC plates. Molar conductance was mea-
sured using ELICO CM-180 conductivity meter. The microwave
assisted reactions were carried out using CEM-Discover (2450
MHz) (Figure 1), wherein this unit, microwaves are generated
by magnetron at a frequency of 2450 MHz having an output
energy range of 100–600 W.
FIG. 1. CEM DISCOVER Microwave Generater.
to carry out by means of customary conventional methods.[23]
Traditionally, Schiff bases have been prepared by refluxing
mixtures of the amine and the carbonyl compound in an organic
solvent, for example, ethanol or methanol[24] and metal com-
plexes by refluxing Schiff’s bases and metal salt in ethanol or
methanol for several hours. Herein, benign, simple, cost effec-
tive and versatile route to Schiff bases and its metal complexes
in good yield have been demonstrated. The study investigates
the synthesis, characterization and biological study of Schiff’s
base derived from 2, 4-diamino toluene and Salicylaldehyde,
as shown in Figure 2, and its complexes by MORE method. In
general, the reactions are very clean, without any side product in
every run. In fact, the crude products obtained are of high purity
with remarkable yields and do not require any chromatographic
separation.
2.3. Synthesis
2.3.1. Synthesis of Schiff’s base ligand N, Nꢀ-bis (salicylidine)
toluene-2, 4-diamine by MWI
The Schiff’s base ligand used in this work, N, Nꢀ-bis (salicy-
lidine) toluene-2, 4-diamine, was obtained by the condensation
of toluene-2, 4-diamine and salicylaldehyde. In a typical sol-
ventless preparation, mixture of diamine (4.92 mmol, 0.60 g)
and salicylaldehyde (9.84 mmol, 1.2 g) without solvent were
taken in a flask capped with funnel placed in a microwave oven
and irradiated at 200 watt for 20 secs. Reaction was monitored
by TLC. After completion of the reaction, the resultant mixture
was allowed to attain room temperature. After cooling, the re-
sultant solid was crushed, washed with cold ethanol, filtered,
and dried under vacuum to give the crude product. The crude
product was recrystallized from methanol.
Within the framework of ‘Green Chemistry’ and continuation
of work with microwaves,[25,26,30] we have now developed an
environmentally benign and novel approach for the synthesis of
Schiff’s base and its metal complexes. The synthesized ligand
and complexes have been screened for in vitro biology against
a panel of gram positive and gram negative bacteria.
OH
OH
Yield: 92%. M.P.120◦C. Anal. calc. for: C21H18N2O2, C,
76.30; H, 5.40; N, 8.45; O, 10.10 %. Found: C, 76.38; H, 5.45; N,
N
N
8.52; O, 9.80 %. λmax (DMF/nm), 345, 265 and 245. IR (cm−1
,
1
KBr): 1620 ( C=N), 1285 ( C=O), 3210 ( OH). H-NMR
(ppm, DMSO-d6): 2.3 (s, 3H, CH3), 8.8 (s, 2H, CH=N),
6.7-7.4 (m, 11H, C6H5), 13.2 (s, 2H, OH). MS (m/z) 330
(100%), 331 (45%).
H
H
CH3
FIG. 2. Schiff’s base ligand.

BENIGN METHODOLOGIES FOR COORDINATION
635
2.3.2. Synthesis of metal complexes by MWI
2.4.4. Preparation of base-layer medium
Base-layer medium was prepared by dissolving definite vol-
N, Nꢀ-bis (salicylidine) toluene-2, 4-diamine (3.0 mmol,
1.0g) was added to ethanol (10 mL) solution of Zn (CH3COO)2 umes of peptone (0.6%), yeast extract (0.3%), beef extract
(3.0 mmol., 0.6g). The mixture was taken in a flask capped with (0.13%), and agar (2.5%) in distilled water. The pH of this
funnel placed in a microwave oven and irradiated at 600 watt for medium was also adjusted to 7.2 and sterilized by autoclaving
5.0 to 6.0 minutes maintaining the occasional shaking. Reaction at 15 p.s.i. for 20 minutes.
was monitored by TLC for the consumption of ligand. After
2.4.5. Preparation of test compounds
completion of reaction, the hot reaction mixture was filtered,
washed with hot ethanol and ether followed by drying in an
oven. For the preparation of other complexes of Cu, Co, Ni and
Mn, the same process was followed.
Each test compounds (5 mg) was dissolved in dimethylfor-
mamide (5 mL) to give a solution of 1000 µg/mL. Out of this
0.1mL. of solution was used for antimicrobial testing.
Yield: 89%. M.P. > 250◦C. Anal. calc. for: C46H38N4ZnO8,
C, 61.01; H, 4.23; N, 6.19; O, 14.13 %. Found: C, 61.03; H, 4.25; 2.4.6. Testing method
N, 6.21; O, 14.15 %. λmax (DMF/nm), 405.5, 300.5 and 259.5.
IR (cm−1, KBr): 1620 ( C=N), 1300 ( C O), 525 (Zn N) and
445 (Zn O). 1H-NMR (ppm, DMSO-d6): 2.23 (s, 3H, CH3),
9.82 (s, 2H, CH=N), 6.32-7.09 (m, 22H, C6H5). MS (m/z)
803 (100%), 804 (57%).
Base-layer was obtained by pouring about 10–15 mL of base
layer medium into each sterilized petri dishes and were allowed
to attain room temperature. This solid layer after attaining room
temperature is called base-layer. Over-night, grown sub-cultures
of bacteria were mixed with seed layer medium and immedi-
ately poured into petri dishes containing the base-layer and then
allowed to attain room temperature. The cups were made by
scooping out nutrient agar with a sterile cork borer. To these
cups, solutions of test compounds (0.1 mL) were added using
sterile pipettes and these plates were subsequently incubated at
37?C for 36 hours. The zone of inhibitions, if any, was measured
in mm for the particular compound. Amoxicillin was used as
positive-control and solvent-control was also used to know the
activity of the solvent. The results of antibacterial testing are
summarized in Table 6.
2.4. Antibacterial Activity
Antibacterial activity of test compounds was assessed against
four test micro-organisms, i.e., P. aureginosa, E. coli, B. subtilis,
and S. aureus by cup-plate method.
2.4.1. Materials
(1) Nutrient agar. (2) Sterilized petri dishes and pipettes.
(3) 20–24-hour old subcultures in nutrient agar medium. (4)
Sterilized test tubes containing solution of the test compounds
in desired concentration.
3. RESULTS AND DISCUSSION
Schiff’s base forms complexes with metal ions via N and
O donor atoms. These N and O atoms induce two opposite
electronic effects: the phenolate oxygen is a hard donor, which
stabilizes the higher oxidation states, while the imine nitrogen
is a softer donor and will hence stabilize the lower oxidation
states of the metal ion.[27] Schiff’s base ligand N, Nꢀ-bis (sali-
cylidine) toluene-2, 4-diamine was prepared from the reaction
of toluene-2, 4-diamine and salicylaldehyde using MORE (mi-
crowave induced organic reaction enhancement) technique, as
shown in Scheme 1. Complexes were readily synthesized by
2.4.2. Preparation of inoculation medium
The definite volumes of peptone (0.5%), yeast extract
(0.15%), beef extract (0.15%), sodium chloride (0.35%),
dipotassium phosphate (0.36%), and potassium dihydrogen
phosphate (0.13%) were dissolved in distilled water and the pH
was adjusted to 7.2. This solution was sterilized by autoclaving
at 15 p.s.i. for 20 minutes.
2.4.3. Preparation of subcultures
One day prior to these tests, inoculation of above bacterial reaction of various metal acetates of Zn, Cu, Co, Ni and Mn
cultures was made in the inoculation medium as described above with the N, Nꢀ-bis (salicylidine) toluene-2, 4-diamine ligand by
and incubated at 37◦C for 18–24 hours.
MORE method, as shown in Scheme 1.
OH
H3C
N
CHO
N
CH
M(CH3COO)2,EtOH,
H2N
NH2
CH3
II
MW, 200 W, 20 S
M2 [ Sal-2,4-Toluene ]2.(OAc)2
CH
MW, 600 W, 5-6 Min
OH
III
OH
IV
I
SCH. 1.

636
P. ALI ET AL.
TABLE 1
Physical characterization and comparative yield of the ligand and the complexes
MORE method
Molar conductance
Compounds
Molecular formula
Color
M.P
Time (min)
% Yield
(ohm−1cm2mol−1
)
H2Sal-2,4-toluene
C21H18N2O2
Yellow
Brown
Red
Brown
Brown
Orange
120
>250
>250
>250
>250
>250
0.30
5.0
5.5
6.0
5.0
5.5
92
89
95
88
90
91
—
56
80
75
87
69
Zn2(sal-2,4-toluene)2
Co2(sal-2,4-toluene)2
Cu2(sal-2,4-toluene)2
Ni2(sal-2,4-toluene)2
Mn2(sal-2,4-toluene)2
C42H32N4Zn2O4
C42H32N4Co2O4
C42H32N4Cu2O4
C42H32N4Ni2O4
C42H32N4Ni2O4
In microwave method, the yield of Schiff base N, Nꢀ-bis
(salicylidine) toluene-2, 4-diamine and its transition metal com-
plexes are more than good as compared to the synthesis by
reported classical method with less consumption of solvent.
Microwave irradiation method facilitates the polarization of the
reacting molecule under the irradiation causing a fast reaction to
occur. When compared to thermal heating, microwave heating
has many benefits in organic synthesis, such as rapid volumetric
heating, higher reaction rates, higher reaction selectivity, higher
product yield, and energy saving. In microwave heating, the en-
ergy source of the microwaves is not in direct contact with the
reaction solution, so the heat produced is via dielectric heating.
This can lead to completely different temperature profiles. The
Schiff’s base ligand and complexes are characterized by FTIR,
MS, 1H-NMR spectroscopy, and elemental analysis.
the ligand and complexes together with some physical properties
are summarized in Table 1. The data from complexes correspond
well with the general formula [M2 (sal-2, 4-toluene)2] (M=
Cu, Zn, Co, Ni, and Mn). Elemental analysis of ligand N, Nꢀ-
bis (salicylidine) toluene-2, 4-diamine and its complexes, as
shown in Table 2, show good agreement with the proposed
structure. Hence, it is clear from all the tabular data that the
MORE technique for the synthesis of the complexes has been
found to be high yielding, easier, convenient, and ecofriendly as
compared with the classical method of synthesis.
3.1. IR Spectral Studies
IR spectral data of the compounds and their assignments are
shown in Table 3. The IR spectra of the complexes were com-
pared with the spectrum of the free ligand in order to determine
the coordination sites that may be involved in complexation.
There were some guide peaks in the spectrum of the ligand,
which were helpful in achieving this goal. The position and the
intensities of these peaks are expected to change upon complexa-
tion. These guide peaks are shown in Table 3. In the spectra of the
ligand, N, Nꢀ-bis(salicylidine)toluene-2,4-diamine (H2Sal-2,4-
toluene), the strong absorption peak around 3000–3200 cm−1
is due to the phenolic OH group[28] and the characteristic ab-
sorption at 1606–1620 cm−1 and 1283 cm−1 can be assigned to
( C=N) azomethine linkage and phenolic C−O, respectively.
The IR spectrum of the ligand N, Nꢀ-bis (salicylidine) toluene-2,
where, M is Zn, Co, Cu, Ni, and Mn.
In the FTIR spectra of complexes, the bands of C=N (about
1600 cm−1 in the ligand) are shifted to higher frequencies around
1620 cm−1 due to the coordination between azomethine nitrogen
and the central metal. The molecular structure of the complexes
has been determined by X-ray diffraction technique and proven
by the mass spectrometry. It consists of dimeric centrosymmeric
units, with two bis (salicylidineiminito) residues bridged by
two methyl-substituted m-phenylene groups. The complexes are
air-stable, non-hygroscopic, colored solids, insoluble in water,
soluble in DMSO, chloroform, and DMF. The important data for
TABLE 2
Elemental analysis of ligand and complexes
Found (calcd.) %
Compound
C
H
N
O
H2Sal-2,4-toluene
76.33(76.30)
61.03(61.01)
61.91(61.89)
61.28(61.26)
65.40(61.92)
62.47(62.45)
5.45(5.40)
4.25(4.23)
4.31(4.29)
4.27(4.25)
4.50(4.29)
4.35(4.33)
8.48(8.45)
6.21(6.19)
6.30(6.28)
6.23(6.21)
6.30(6.28)
6.35(6.33)
9.69(10.10)
14.15(14.13)
14.36(14.34)
14.21(14.19)
14.37(14.35)
14.49(14.47)
Zn2(sal-2,4-toluene)2
Co2(sal-2,4-toluene)2
Cu2(sal-2,4-toluene)2
Ni2(sal-2,4-toluene)2
Mn2(sal-2,4-toluene)2

BENIGN METHODOLOGIES FOR COORDINATION
637
TABLE 3
Infrared spectral data V(cm−1) of ligand and complexes
Compound
V (OH)
V(C=N)
V (Phenolic C O)
V (M N)
V (M O)
H2Sal-2,4-toluene
3200
—
—
—
—
1618
1620
1610
1606
1608
1618
1283
1300
1307
1303
1298
1291
—
—
Zn2(sal-2,4-toluene)2
Co2(sal-2,4-toluene)2
Cu2(sal-2,4-toluene)2
Ni2(sal-2,4-toluene)2
Mn2(sal-2,4-toluene)2
525
521
531
530
528
445
450
430
435
434
—
4-diamine also showed a broad and a weak band in the region
2800–2700 cm−1, which confirms the intra molecular hydrogen
bonded OH.
3.2. UV-Visible Spectral Studies
The UV-VIS spectrum of ligand N, Nꢀ-bis (salicylidine)
toluene-2, 4-diamine (H2Sal-2, 4-toluene) and its complexes
were recorded at room temperature in DMSO. The spectral data
of the compounds are summarized in Table 4. The spectrum
of Schiff’s base H2Sal-2, 4-toluene exhibits three main peaks at
350, 265, and 255 nm. The first and second peaks were attributed
to benzene π-π∗ and imino π-π∗ transitions, respectively. In the
spectra of the complexes, red-shift has been observed in three
bands. It indicates the coordination of the imino nitrogen to
the metal ions and the influence on the conjugation system be-
cause of the ligation between the ligand and the metal ions.
The Mn (II) complex is dark green in solution and shows an
intense absorption in the visible region at 400 nm assigned to
Compared to H2Sal-2, 4-toluene, some changes have taken
place in the IR spectra of the complexes. First, the strong peak
at 3200 cm−1 disappeared. This suggests deprotonation of the
phenolic OH group in H2Sal-2, 4-toluene because of the co-
ordination with the metal ions. Second, the absorption peaks
of azomethine linkage C=N shifted by 10–30 cm−1 and this
indicates the possibility of coordination of the imino nitrogen
to the metal ions.
Thus, the IR spectra of the complexes (Table 3) show a shift
of the (C-O Phenol) band to higher frequency (10–25 cm−1
)
and a negative shift of the ( C=N) band (10–30 cm−1) with
respect to the free ligand. These data suggest the coordination
of the Schiff base N, Nꢀ-bis (salicylidine) toluene-2, 4-diamine
in its dianionic form to the metal through the phenol oxygen and
the imine nitrogen atoms, and show the tetra dentate character
of the ligand in the complexes. Furthermore, some new bands
have been observed between 420 and 530 cm−1. The bands in
the range 500–531cm−1are attributed to M−N=C and those
sharp absorption bands in the region 630–670 cm−1 are the
characteristic of a dimeric core. These bands do not appear in the
mononuclear complexes. The number and shapes of the bands
in all dimers also confirm that Schiff’s base ligand N, Nꢀ-bis
(salicylidine) toluene-2, 4-diamine adopt a bridging disposition
between the two metal centers.
4
6A1 → T1 transition, suggesting a tetrahedral geometry for the
Mn (II) complex. The electronic spectrum of the red Co (II)
complex exhibits a band at 400.5 nm assigned to 1A1g → B1g
1
for square-planar geometry. Square-planar Ni (II) complexes are
commonly orange and shows absorption in the visible region at
400nm with a second band at 280 nm. These bands are com-
1
1
1
1
monly assigned to A1g → A2g (b2g →b1g) and A1g → B1g
(a1g →b1g), respectively. The electronic spectrum of Zn (II)
complex exhibits intensity at 405.5 nm due to ligand–metal
charge transfer (LMCT).[29]
3.3. 1H-NMR Spectral Studies
1
The H-NMR data and assignments of the compounds are
1
presented in Table 5. The H-NMR spectrum of the ligand N,
Nꢀ-bis (salicylidine) toluene-2, 4-diamine (H2Sal-2,4-toiuene)
in DMSO-d6 at room temperature using TMS as an internal
standard showed the following signals: The Ar CH3 proton
at 2.3 ppm (s, 3H), CH=N at 8.8 ppm (s, 2H) and phenyl
as multiplet at 6.7–7.4 ppm (m, 11). Furthermore, the peak
obtained at 13.2 ppm (s, 2H) is attributable to phenolic OH
TABLE 4
UV-visible spectral data of the Schiff’s base and its complexes
Compounds
Absorption (λmax) in lmol−1cm−1
H2Sal-2,4-toluene
350.0
405.5
400.5
410.0
400.0
400.0
265.0
300.5
280.5
285.0
280.0
287.0
255.0
259.5
258.0
265.0
260.0
268.0
1
group present in 2- hydroxyl benzaldehyde moiety. The H-
Zn2(sal-2,4-toluene)2
Co2(sal-2,4-toluene)2
Cu2(sal-2,4-toluene)2
Ni2(sal-2,4-toluene)2
Mn2(sal-2,4-toluene)2
NMR spectrum of the ligand and its zinc complex were recorded
in DMSO-d6. The 1H NMR spectrum of the complex shows the
following signals: Ar CH3 proton at 2.23 ppm (s, 6H), phenyl
as multiplet at 6.32–7.09 ppm (m, 22H) and CH=N at 9.82
ppm (s, 4H). The absence of phenolic OH proton noted for

638
P. ALI ET AL.
TABLE 5
1H-NMR spectral data (ppm) of the Schiff’s base and its complexes
Compounds
OH
Aromatic H
C (H)=N
CH3
H2Sal-2,4-toluene
13.25 (s, 2H)
6.72–7.45 (m, 11H)
6.32–7.09 (m, 22H)
6.32–7.00 (m, 22H)
6.33–7.07 (m, 22H)
6.33–7.08 (m, 22H)
6.34–7.05 (m, 22H)
8.82 (s, 2H)
9.82 (s, 4H)
9.82 (s, 4H)
9.83 (s, 4H)
9.83(s, 4H)
9.81 (s, 4H)
2.33 (s, 3H)
2.23 (s, 6H)
2.22 (s, 6H)
2.25 (s, 6H)
2.25 (s, 6H)
2.23 (s, 6H)
Zn2(sal-2,4-toluene)2
Co2(sal-2,4-toluene)2
Cu2(sal-2,4-toluene)2
Ni2(sal-2,4-toluene)2
Mn2(sal-2,4-toluene)2
—
—
—
—
—
the Zn complex confirms involvement of the OH proton in
complexation. The azomethine proton signal in the spectrum
of the Zn complex is shifted downfield compared to the free
ligand, suggesting deshielding of the azomethine group due to
coordination with metal ion. There is no appreciable change in
all the other signals of this complex.
3.5. X-ray Diffraction Studies
When N, Nꢀ-bis (salicylidine) toluene-2, 4-diamine (H2Sal-
2, 4-toluene) is exposed to X-ray diffraction analysis, it gives
a diffractogram with five peaks characteristic of crystalline re-
gions at 14.83, 14.91, 15.42, 17.26, and 17.38. But Co2 (sal-2,
4-toluene)2 gives a diffractogram without characteristic peak
of crystalline region, Cu2 (sal-2, 4 toluene)2 gives a diffrac-
togram with peaks at 5.87, 8.33, 8.91, 9.36, and 10.57. Zn2 (sal-
2, 4-toluene)2, and Ni2 (sal-2, 4-toluene)2 also give a diffrac-
togram with some peaks. The X-ray diffractograms of H2Sal-2,
4-toluene, and its metal complexes support that H2Sal-2, 4-
toluene has higher crystallinity than its complexes.
3.4. Mass Spectral Studies
The mass spectrum of the ligand N, Nꢀ-bis (salicylidine)
toluene-2, 4-diamine (H2Sal-2, 4-toluene) and its copper com-
plex Cu2 (Sal-2, 4-toluene)2 recorded at room temperature were
used to compare their stoichiometry composition. The Schiff
base showed a molecular ion peak at 330 (100 %), which was
also supported by the ‘nitrogen rule’, since the compound pos-
sess two nitrogen atoms and other peaks at 331 (23%) and 332
(3.1%). The molecular ion peak for the copper complex ob-
served at 811 (100%) and other peaks at 334 (94 %) and 332
(50 %) confirms the stoichiometry of metal chelates as M2L2
(Dimer) type. It is also supported by the mass spectral data of
other complexes. Based on these data, structure, as shown in
Figure 3, has been proposed.
3.6. Molar Conductivities of the Complexes
Conductivity measurements in chloroform provide a method
of testing the degree of ionization of the complexes. The con-
ductivity data reported for 10−3 M solution for these complexes
are given in Table 1. It is clear from the conductivity data (higher
conductivity values) that the complexes are electrolytes. Thus,
CH3COO− is present, and completes the coordination sphere.
On the basis of physical and spectral data, following structure
is proposed for Schiff’s base metal complexes.
TABLE 6
Antibacterial screening data of the ligand and complexes
n+
Gram Positive
Gram Negative
O
O
Compounds
S.aureus B.subtilis P.aeruginosa E.coli
M
M
HC
CH
N
H2Sal-2,4-toluene
Zn2(sal-2,4-toluene)2
+
+
+
++
++
+
—
—
+
+
N
CH3
n(CH3COO)-
Co2(sal-2,4-toluene)2 + + +
++
+ + +
+
Cu2(sal-2,4-toluene)2
Ni2(sal-2,4-toluene)2
+
—
—
+
+
+
H3C
N
N
Mn2(sal-2,4-toluene)2 ++
+
++
+ + +
—
+
HC
CH
Amoxicillin
+ + + + + +
+ + +
—
Dimethylformamide
—
—
O
O
Key to symbols: Inactive = –(inhibition zone < 5 mm); slightly
active = + (inhibition zone 5–10 mm); moderately active = ++
(inhibition zone 10–15 mm); highly active = + + + (inhibition zone
> 15 mm).
FIG. 3. Proposed structure for the synthesized metal complexes.

BENIGN METHODOLOGIES FOR COORDINATION
639
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506.
11. Sharma, K.V., Sharma, V., Dubey, R.K., and Tripathi, U.N. J. Coord.
Chem.,2009, 62(3), 493.
12. Nakasuka, N., Kunimatsu, M., Matsumura, K., and Tanaka, M. Inorg.
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3.7. Antimicrobial Studies
In the light of interesting antimicrobial activities of the coor-
dination complexes, the ligands and their corresponding com-
plexes were screened for antibacterial activity against two gram
positive strains of S. Aureus, B. subtilis, and two gram negative
strains of P. aeruginosa, E. coli, respectively, by the cup plate
method using Nutrient agar. The screening data of the inhibition
of the bacteria are given in Table 6. From the data, it is clear that
the free ligand is slightly against the test micro-organisms. On
complexation, there is a notable enhancement of antibacterial
activity. The promising results were observed for the Co (II)
complex against all the test bacteria.
15. Nakasuka, N., Azuma, Sh., and Tanaka, M. Acta Crystallogr. Sect.C, 1986,
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4. CONCLUSION
In this communication, we have reported the synthesis of
Schiff’s base ligand derived from salicylaldehyde and toluene
2, 4-diamine using MORE technique. Herein, benign, simple
and versatile routes to bis-imine Schiff’s base and its metal
complexes in good yield have been demonstrated. The MORE
method for the synthesis of complexes has been found easier,
convenient, and ecofriendly as compared to the reported clas-
sical method by Mederos. From the elemental analysis, molar
1
conductivity, UV-visible, mass, IR, H-NMR, and XRD spec-
tral data it was possible to determine the type of coordination
of the ligand in their metal complexes. Data have shown the
formation of dimer complexes, where the ligand acts as a bridge
between two metal cations. The complex especially Co (II) has
shown good antibacterial properties against the tested strains of
bacteria.
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