Design, synthesis and characterization of macrocyclic ligand based transition metal complexes of Ni(II), Cu(II) and Co(II) with their antimicrobial and antioxidant evaluation

Article abstract of DOI:10.1016/j.molstruc.2017.01.033

Three new complexes Ni(II), Cu(II) and Co(II) were synthesized of macrocyclic ligand derived from 1, 4-dicarbonyl-phenyl-dihydrazide and O-phthalaldehyde in the ratio of 2:2. The synthesized compounds were characterized by elemental analyses, molar conductance, magnetic susceptibility measurements, FTIR, UV–Vis., Mass and ¹H NMR spectral studies. The electronic spectra of the metal complexes indicate a six coordinate octahedral geometry of the central metal ion. These metal complexes and the ligand were evaluated for antimicrobial activity against bacteria (E.?coli, B.?subtilis, S.?aureus) and fungi (A.?niger, A.?flavus, C.?albicans) and compared against standard drugs chloramphenicol and nystatin respectively. In addition, the antioxidant activity of the compounds was also investigated through scavenging effect on DPPH radicals.

Full text of DOI:10.1016/j.molstruc.2017.01.033

Accepted Manuscript
Design, synthesis and characterization of macrocyclic ligand based transition
metal complexes of Ni(II), Cu(II) and Co(II) with their antimicrobial and antioxidant
evaluation
Parveez Gull, Manzoor Ahmad Malik, Ovas Ahmad Dar, Athar Adil Hashmi
PII:
S0022-2860(17)30053-4
10.1016/j.molstruc.2017.01.033
MOLSTR 23338
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 30 September 2016
Revised Date: 5 January 2017
Accepted Date: 9 January 2017
Please cite this article as: P. Gull, M.A. Malik, O.A. Dar, A.A. Hashmi, Design, synthesis and
characterization of macrocyclic ligand based transition metal complexes of Ni(II), Cu(II) and Co(II) with
their antimicrobial and antioxidant evaluation, Journal of Molecular Structure (2017), doi: 10.1016/
j.molstruc.2017.01.033.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Design, synthesis and characterization of macrocyclic ligand based transition metal
complexes of Ni(II), Cu(II) and Co(II) with their antimicrobial and antioxidant
evaluation.
Parveez Gull*, Manzoor Ahmad Malik, Ovas Ahmad Dar, Athar Adil Hashmi*
Department of chemistry, Jamia Millia Islamia, New Delhi, India 110025
*Corresponding Address:
Prof. Athar Adil Hashmi and Dr. Parveez Gull
Department of Chemistry
Jamia Millia Islamia (Central University)
Jamia Nagar, New Delhi - 110025, India
Tel: 0091-11-26981717 Ext. 3269
E. mail: dr.aahashmi@yahoo.co.in
parveezgull@gmail.com
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Abstract
Three new complexes Ni(II), Cu(II) and Co(II) were synthesized of macrocyclic ligand
derived from 1, 4-dicarbonyl-phenyl-dihydrazide and O-phthalaldehyde in the ratio of 2:2.
The synthesized compounds were characterized by elemental analyses, molar conductance,
1
magnetic susceptibility measurements, FTIR, UV–Vis., Mass and H NMR spectral studies.
The electronic spectra of the metal complexes indicate a six coordinate octahedral geometry
of the central metal ion. These metal complexes and the ligand were evaluated for
antimicrobial activity against bacteria (E. coli, B. subtilis, S. aureus) and fungi (A. niger, A.
flavus, C. albicans) and compared against standard drugs chloramphenicol and nystatin
respectively. In addition, the antioxidant activity of the compounds was also investigated
through scavenging effect on DPPH radicals.
Keywords: Metal complexes, Macrocycle, ligand, Spectroscopic study, Antimicrobial
activity, Antioxidant activity
1. Introduction
In the last two decades coordination chemistry of macrocyclic ligands have shown a
considerable interest [1, 2]. The use of macrocyclic ligands as models for protein-metal
binding sites in biological systems, such as the synthetic ionophores, models for the magnetic
exchange phenomena, therapeutic reagents in chelate therapy for treatment of metal
intoxication and the cyclic antibiotics that retain their antibiotic actions to specific metal
complexation [3–6] raises the importance of new macrocyclic ligand designing. The great
importance of macrocyclic systems in chemistry is due to their selective chelation towards
certain metal ions depending on the number, type and position of their donor atoms, the ionic
radii of the metal centers, and the coordinating property of the counter ions [7]. The synthetic
macrocyclic complexes mimic the naturally occurring bio-macrocyclic systems such as iron-
porphyrin core in haemoglobin, cobalt-corrin of vitamin B12 and magnesium-hydroporphyrin
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in chlorophyll [8] which boost their importance from the biological point of view. There are a
number of important macrocyclic molecules which show biological activities including
antibacterial, antifungal, antidiabetic, antiproliferative, anticancer, herbicidal and anti-
inflammatory activities [9-16].
Microbial diseases have become a threat even in the current century being still very
much problematic to diagnose. During the last century various types of antibacterial and
antifungal drugs have been discovered [17-19]. Presently antibacterial sulfa drugs,
nitrofuranes, penicillins, cephalosporins, tetracyclines and oxaolidin and antifungal agents
such as fluconazole, ketoconazole and amphotericin B are used as antimicrobial agents [20-
23]. There are still various problems unsolved for various antimicrobial agents present till
date. Fungal infections are not only limited to superficial tissues but are also reported for the
life threatening level [24]. The main reason for this serious level may be owing to the huge
number of patients at risk, including those with progressive age, major surgery,
immunosuppressive therapy, acquired immunodeficiency syndrome (AIDS), cancer treatment
and solid-organ and hematopoietic stem cell transplantation [25]. To develop the more
effective antimicrobial drugs is the need of an hour and the macrocyclic metal complexes are
also known to act as promising antimicrobial agents [26-29].
In this manuscript, the synthesis, spectral characterization and biological properties of
the new Ni (II), Cu (II) and Co (II) complexes of 1, 4-dicarbonyl-phenyl-dihydrazide with O-
phthalaldehyde have been described to shed more light on the structure and biological
properties of this new series of metal complexes.
2. Experimental
2.1. Reagents
All chemicals used were of the analytical grade. Diethyl terephthalate, O-phthalaldehyde, and
hydrazinhydrate were procured from Aldrich. Metal salts were purchased from Merck.
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2.2. Synthesis of 1, 4-dicarbonyl-phenyl-dihydrazide
Diethyl terephthalate (2.22 g) was dissolved in 30 mL mixture of ethanol. The solution of
diethyl terephthalate was added to a solution of hydrazine hydrate (98% 2 mL) dissolved in
20 mL ethanol. After complete addition, the solution was stirred for 4 h. After stirring, the
solvent was removed by rotary evaporation under reduced pressure. The resulting dihydrazide
of terephthalic acid was collected, washed with water and then recrystallized from methanol
and dried in vacuum [30].
2.3. Synthesis of macrocyclic ligand
A solution of 0.27 g of O-phthalaldehyde (2 mmol) in 25 ml ethanol was reacted with 0.39 g
(2 mmol) of 1,4-dicarbonyl-phenyl-dihydrazide in 30 ml ethanol in a round bottom flask.
Stirring was continued for 3-4 h at room temperature during which a solid compound
separated.
It
was
filtered,
washed
with
methanol,
recrystallized
from
dichloromethane/methanol and dried in vacuum. (Scheme 1)
1
Melting point 145ºC and Yield 65-70% H NMR (400 MHz, DMSO-d6) δ=7.30-7.80 (m,
13
16H, Ar–H), 7.98 (s, Ar-H, carbonyl Phenyl), 9.15 (s, 4x1H, -NH of hydrazide). C NMR
(400 MHz, DMSO-d6) δ = 137.2 (C═N), 170.5 (C═O), 138.5, 135.2, 130.1, 128.9, 127.4
(Ar–C). UV/vis (1 x 10-4 mol, DMSO): λ (cm-1) = 27777, 25641, 22471. IR (KBr disc, cm-1)
ν(N‒H) 3250, ν(C=N) 1625, ν(C=O) 1720, ν(N–N) 1605, ν(C–C) 1105, ν(C=C, aromatic)
755, ν(C–H, aromatic) 1536.
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2 NH NH .HO
2
2
2
O
O
Hydr az nie h ydr aet
D ei ht yletre ph ht alaet
O
O
H N NH
NH
NH
2
2
1,4- dicarb on y-l phen y-l dhi ydr az ide
C HO H
O
O
2 5
HCl
O- ph ht alaldeh yde
O
O
NH
N
NH
N
N
N
NH
NH
O
O
L
Scheme 1. Synthesis of Macrocyclic ligand.
2.4. Synthesis of the Ni(II), Cu(II) and Co(II) complexes
The metal complexes were synthesised by refluxing a methanolic solution of macrocyclic
ligand (1.0 mmol) and M(Cl)2 metal salts (1.0 mmol) for 2 h during which a precipitate
separated. The precipitate was filtered off, washed with ethanol and dried under vacuum over
anhydrous CaCl2 (65-72% yield) (Scheme 2).
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2.4.1. Synthesis of metal complex (1)
Synthesis of [Ni(C32H24N8O4)Cl2] complex(1). A solution of NiCl2.6H2O in methanol (15
mL) was mixed with the hot solution of the macrocyclic ligand (L) in ethanol (1:1) and the
reaction was refluxed for 6 hours till the precipitate appeared. The precipitate was filtered off,
washed with methanol and dried under vacuum over anhydrous CaCl2 Yield 72%. UV/vis (1
x 10-4 mol, DMSO): λ = 21097 (3A2g (F) → 3T1g (P)), 13793(3A2g (F) → 3T1g (F)), 12254(3A2g
(F) → 3T2g (F)). IR (KBr disc, cm-1) ν(N‒H) 3235, ν(C=N) 1635, ν(C=O) 1720, ν(N–N) 1612,
ν(C–C) 1105, ν(C=C, aromatic) 755, ν(C–H, aromatic) 1536, ν(M‒N) 434, ν(M‒Cl) 316.
2.4.2. Synthesis of metal complex (2)
Synthesis of [Cu(C32H24N8O4)Cl2] complex (2). A solution of CuCl2.2H2O in methanol
(15mL) was mixed with the hot solution of the macrocyclic ligand (L) in ethanol (1:1) and
the reaction was refluxed for 6 hours till the precipitate appeared. The precipitate was filtered
off, washed with methanol and dried under vacuum over anhydrous CaCl2 Yield 68%.
UV/vis (1 x 10-4 mol, DMSO): λ = 15797 (2Eg→2T2g . IR (KBr disc, cm-1) ν(N‒H) 3230,
)
ν(C=N) 1640, ν(C=O) 1720, ν(N–N) 1612, ν(C–C) 1105, ν(C=C, aromatic) 755, ν(C–H,
aromatic) 1536, ν(M‒N) 450, ν(M‒Cl) 310.
2.4.3. Synthesis of metal complex (3)
Synthesis of [Co(C32H24N8O4)Cl2] complex (2). A solution of CoCl2.6H2O in methanol
(15mL) was mixed with the hot solution of the macrocyclic ligand (L) in ethanol (1:1) and
the reaction was refluxed for 6 hours till the precipitate appeared. The precipitate was filtered
off, washed with methanol and dried under vacuum over anhydrous CaCl2 Yield 73%.
UV/vis (1 x 10-4 mol, DMSO): λ = 19047 4T1g (F) →4A2g (F)), 14450 (4T1g (F) →4T1g (P)) IR
(
(KBr disc, cm-1) ν(N‒H) 3225, ν(C=N) 1630, ν(C=O) 1720, ν(N–N) 1612, ν(C–C) 1109,
ν(C=C, aromatic) 757, ν(C–H, aromatic) 1535, ν(M‒N) 440, ν(M‒Cl) 318.
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O
O
NH
NH
N
N
N
N
NH
NH
O
O
MCl2
O
O
NH
NH
N
N
Cl
M
Cl
N
N
NH
NH
O
O
Scheme 2. Synthesis of Macrocyclic metal complexes. (M = Ni, Co, Cu)
3. Pharmacology
3.1. Antimicrobial activity
The antimicrobial activities of the synthesised compounds (macrocyclic ligand and its metal
complexes) have been screened in vitro, as growth inhibiting agents. The antifungal and
antibacterial screening were carried out using Food Poison and Disc Diffusion Method
against some strains of bacteria and fungi like E. coli, B. subtilis, and S. aureus and A. niger,
A. flavus and C. albicans respectively. The compounds were dissolved in DMSO (1%) to get
the required test solutions. The PDA and nutrient agar were used as a required medium for
these activities. After incubation for 24 h at 27°C in the case of bacteria and for 7 days at
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27°C in the case of fungi, the inhibition of the organisms evidenced by clear zone surround
each disk was measured.
3.2. Antioxidant activity
The antioxidant activity of the compounds is evaluated by using the scavenging ability of the
stable DPPH (2, 2׳-diphenyl-1-picryl hydrazyl) radical. The scavenging capacity of DPPH•
was determined spectrophotometrically.
4. Results and discussion
The general composition for all the complexes was found to be MLX2 (where M = Ni, Cu
and Co; L = C32H24N8O4 and X=Cl-). All the complexes provide proper C, H and N results
which were in good agreement with those calculated for the suggested formulae. The results
of elemental analyses with molecular formulae were listed in Table 1. The complexes
obtained were variously colored, polycrystalline, air stable and insoluble in common organic
solvents but readily soluble in DMSO and DMF.
4.1. Molar conductance measurements
The molar conductivities 10-3 M solutions of the synthesised metal Complexes in DMF were
studied at room temperature. The complexes showed molar conductance values in the range
of 12–18 ꢀ-1cm2mol-1 showing that complexes were non-electrolytic in nature [31,32]. The
molar conductance value suggested that the complexes may be formulated as [MLX2] and the
anions were inside the coordination sphere and bonded to the central metal ion.
4.2. Magnetic susceptibility measurements
The Ni(II) complexes showed magnetic moment value of 3.3 B.M. corresponding to the
range normally observed for octahedral geometry [33]. The Cu(II) complexes showed
magnetic moment in the range of 1.92–1.99 B.M., corresponding to one electron that falls
within the range normally observed for six coordinated Cu(II) complexes [34]. The magnetic
moment values of the Co(II) complexes were reported in the range of 4.72–4.95 B.M. which
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is close to the normal range for octahedral complexes is (4.3– 5.2 B.M.), hence indicative of
octahedral geometry [35,36]. The experimental values were higher than the spin only value
due to orbital angular momentum contribution in d7 system.
Table 1. Analytical data of the Cu(II), Co(II), and Ni(II) complexes.
Elemental analysis found (calculated) (%)
Complexes
Mol. wt.
584.58
714.18
719.04
714.42
µ eff
C
H
N
M
C32H24N8O4 (L)
65.75 (65.69) 4.14(4.08) 19.17(19.11)
-
[Ni(C32H24N8O4)Cl2]
[Cu(C32H24N8O4)Cl2]
[Co(C32H24N8O4)Cl2]
53.82 (53.77) 3.39(3.30) 15.69(15.58) 8.22 (8.18)
3.3
53.45 (53.38) 3.36(3.28) 15.58(15.49) 8.84 (8.76) 1.92-1.99
53.80 (53.75) 3.39(3.31) 15.68(15.59) 8.96 (8.91) 4.72-4.95
4.3. IR spectra
The infrared spectrum which affords valuable evidence about the coordination of the
synthesised compounds with the metal salts has been presented in Figure S1 (Supplementary
Data). The absence of bands ~Ca. 3400 cm-1 characteristic of the free –NH2 group supported
the formations of macrocyclic ligand by the condensation between 1, 4-dicarbonyl-phenyl-
dihydrazide and O-phthalaldehyde molecules. The occurrence of a medium intensity band at
ca. 1550–1640 cm-1 attributable to the imine ʋ(C=N) stretching frequency [37,38], which
appeared at lower frequency values in the complexes, which indicates that the coordination
has took place through the C=N group. The appearance of a band at ca. 430–470 cm-1 in the
spectra of the complexes, which was assigned to ʋ(M–N) is also an indication of the metal
azomethine nitrogen bond. A medium intensity band at 3310–3250-3225 cm-1 was assigned
to ʋ(N–H) stretching mode of ‒NHCO– moiety [39]. In the far IR region, the ʋ(M–Cl) and
ʋ(M–N) bands appeared in the 310–330 and 420–445 cm-1 [40,41].
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4.4. Mass spectra
Mass spectra provided additional structural information about the analysed species. The ESI
mass spectra of the synthesised ligand C32H24N8O4 (L) exhibited the molecular ion peak at
m/z 584.13 as shown in figure S2. The electronic impact mass spectrum of the Co(II) choro
complex showed a molecular ion [Co(C32H24N8O4)Cl2]+ peak at m/z 715.06 amu (figure S3)
and the molecular ion peak may be corresponding to isotopic peak (M+1) due to 13C and 15N
isotopes. Cu(II) complex showed molecular ion [Cu(C32H24N8O4)Cl2]+ peak at m/z 720 amu
and the Ni(II) complex at 715.18 which corresponds to the molecular ion peak of
[Ni(C32H24N8O4)Cl2]+. In addition to the peaks due to the molecular ion, the spectra exhibit
peaks assignable to various fragments arising from the thermal cleavage of the complexes
[42]. The intensity of these peaks gave the idea regarding the stability and abundance of the
ions. This data is in good agreement with the proposed molecular formula for these
complexes.
4.5. 1H NMR
The 1H NMR spectra of macrocyclic ligand (L) (Figure S4) was recorded in
1
dimethylsulfoxide (DMSO d6) solution using Me4Si (TMS) as internal standard. The H
NMR spectra of the ligand shows broad signal at 9.15 ppm due to the ‒NH [43,38], 7.98 ppm
due to the ‒CH=N [44]. The multiplets in the region 7.30-7.80 ppm may be assigned to
aromatic proton [45]. 13C NMR spectra of the macrocyclic ligand and its metal complexes are
recorded in dimethylsulfoxide (DMSO-d6) solution, using tetramethylsilane (TMS) as
internal standard. 13C NMR spectrum of the symmetrical macrocyclic ligand displayed
characteristic signals at δ 138.5 ppm, 135.2 ppm, 130.1 ppm, 128.9 ppm, 127.4 ppm
corresponding to the aromatic carbons. The signal at 137.2 ppm and 170.5 ppm are due to
13
C═N, and C═O respectively [46]. The C NMR spectra of complexes show no appreciable
changes compared with free ligand.
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4.6. Electronic spectra
The electronic spectra of complexes were recorded in DMSO (Table 2). The spectrum of
Ni(II) complex in DMSO exhibits three bands at 21,097, 13,793 and 12,254 cm-1 assigned to
3
3
3A2g(F)→3T1g(P), A2g(F)→3T1g(F) and A2g(F)→3T2g(F) transitions, respectively, in an
octahedral geometry around Ni(II) ion [47]. The spectrum shows also, a band at 33,003 cm-1
assigned to L→M charge-transfer.
The electronic spectrum of the macrocyclic copper complex shows broad band absorption in
2
2
the range 15,797 cm-1, which may be assign to Eg → T2g transition and it is in conformity
with octahedral geometry [48,49], an indication of the most probable geometric configuration
of the synthesized metal complexes is their magnetic moment values. (Figure 1) The Co(II)
complexes shows electronic spectral bands at 19047 cm-1 and 14450 cm-1 which were
4
4
assigned to the transitions T1g (F) →4A2g (F) and T1g (F) →4T1g (P) respectively. The
positions of bands indicated that Co(II) complexes have an overall octahedral geometry [50].
(Figure 2)
Table 2. Spectral absorption bands of synthesised macrocyclic metal complexes.
Compound
Band position (cm-1)
Assignment
Geometry
[Ni(C32H24N8O4)Cl2]
21097
13793
12254
15797
3A2g (F) → 3T1g (P) Octahedral
3A2g (F) → 3T1g (F)
3A2g (F) → 3T2g (F)
[Cu(C32H24N8O4)Cl2]
[Co(C32H24N8O4)Cl2]
2Eg→2T2g
Octahedral
19047
14450
4T1g (F) →4A2g (F) Octahedral
4T1g (F) →4T1g (P)
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1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
[Cu(C32H24N8O4)Cl2]
[Ni(C32H24N8O4)Cl2]
200
300
400
500
600
700
800
900
Wavelength (nm)
Figure 1. Electronic spectra of Nickle and Copper metal complexes
1.0
[Co(C32H24N8O4)Cl2]
0.8
0.6
0.4
0.2
0.0
-0.2
200
300
400
500
600
700
800
Wavelength (nm)
Figure 2. Electronic spectra of Cobalt metal complexes
4.7 X-ray diffraction analysis
The power X-ray diffraction patterns of the synthesised macrocyclic complexes were used for
the cell refinement and pattern indexing of unit cells. The diffractogram reveals the
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crystalline nature of the metal complexes and are worked out by using trial and error method
for finding the best fit between observed and calculated values [51]. The powder XRD
analysis of the macrocyclic complexes was recorded in the 10–80º (2θ) range and the results
obtained show well resolved sharp peaks with high intensities. The observed sharp peaks
indicate high degree of crystallinity for the complexes (Figure 3-5). The indexing,
calculations of unit cell parameters, scattering angles (2θ) corresponding to each reflection
and inter-planar spacing (d) along with Miller’s indices were evaluated (Table S1-S3) which
corresponds to triclinic, tetragonal, and monoclinic crystal system for Ni(II), Co(II) and
Cu(II) complexes respectively.
2000
Crystal system: Triclinic
WaveLength: 1.540598
Lattice Parameter: a= 11.45 b= 10.19 c= 11.517
Lattice Parameter: Alpha= 90 Beta= 94.67 Gama=120
1500
1000
500
0
10
20
30
40
50
60
70
80
2 theta
Figure 3. X-Ray diffractogram of [Ni(C32H24N8O4)Cl2]
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1000
800
600
400
200
Crystal system: Tetragonal
WaveLength: 1.540598
Lattice Parameter: a= 15.08 b= 15.08 c= 13.43
Lattice Parameter: Alpha= 90 Beta= 90 Gamma=90
10
20
30
40
50
60
70
80
2 theta
Figure 4. X-Ray diffractogram of [Co(C32H24N8O4)Cl2]
Crystal system: Monoclinic
900
WaveLength: 1.540598
Lattice Parameter: a= 24.19 b= 12.12 c= 8.349
Lattice Parameter: Alpha= 90 Beta= 98.71 Gamma=90
800
700
600
500
400
300
200
10
20
30
40
50
2 theta
Figure 5. X-Ray diffractogram of [Cu(C32H24N8O4)Cl2]
4.8. Antibacterial Activity
The antibacterial activity (Table 3) of the macrocyclic ligand and its complexes were
determined by Disc Diffusion Method [52,53]. The test solutions were prepared by dissolving
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the synthesised compounds in DMSO. 20 mL of the nutrient agar medium was poured on
each petriplates and after it 0.5 mL of test bacterial broth was spread over the medium. From
the test solution 25 ꢁL was applied on the sterile disc having a diameter of 10 mm. after the
solvent evaporation, the discs were placed at the centre of the inoculated petriplates and
sealed with para film. The petriplates were incubated at 27°C for 24 h and the inhibition
zones were measured in millimetres.
Table 3. Antibacterial activities of ligand (L) and its complexes.
Compounds
Bacterial inhibition zone, mm (conc. in µgml-1)
E. coli
B. subtilis
S. aureus
100
-
250
-
500
09
16
18
12
32
100
-
250
06
15
12
11
30
500
08
19
18
16
30
100
-
250
-
500
09
13
16
14
35
C32H24N8O4 (L)
[Ni(C32H24N8O4)Cl2]
[Cu(C32H24N8O4)Cl2]
[Co(C32H24N8O4)Cl2]
Chloramphenicol
06
10
07
32
11
14
09
32
08
05
07
30
-
09
13
11
35
07
08
35
4.9. Antifungal Activity
The fungicidal activity (Table 4) of the synthesised macrocyclic ligand and its metal
complexes was determined in vitro by Food Poison Technique [52, 54]. The required test
solutions were prepared by dissolving the compounds in the DMSO. The required quantity of
this solution was added to PDA medium and poured into the petriplate and the 5 mm mycelial
disc of fungus was placed on the medium. Finally, the petriplates were sealed by the para film
and were kept in the incubator at 27°C for 7 days. The activities were compared with the
activity of the standard fungicide Nystatin. The inhibition of fungal growth expressed in
percentage terms was determined from the growth in the test plates compared to the
respective control plates as given below.
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(C − T)
Inhibition % =
× 100
C
Where, C is the Diameter of fungal growth in the control plate and T the Diameter of fungal
growth in the test plate.
Table 4. Antifungal activities of ligand (L) and its complexes.
Fungal inhibition, % (conc. in µgml-1)
Compounds
A. niger
100
28
A. flavus
100
30
C. albicans
50
17
31
150
32
59
51
52
92
50
15
28
21
29
70
150
39
45
50
57
94
50
12
19
24
13
52
100
18
33
37
21
68
150
31
43
51
35
81
C32H24N8O4 (L)
[Ni(C32H24N8O4)Cl2]
41
39
[Cu(C32H24N8O4)Cl2] 26
[Co(C32H24N8O4)Cl2] 29
49
40
39
46
Nystatin
69
84
89
From the results of the antibacterial and antifungal activity of metal complexes, it is
obvious that the metal complexes exhibit greater antimicrobial activity than that of the free
macrocyclic ligand; this progressive antibacterial activity of the metal complexes, compared
with that of Schiff bases, is conceivably owing to modification in structure due to
coordination, and chelating tends to make metal complexes act as more powerful
bacteriostatic agents, thus inhibiting the growth of the microorganisms. The Overtone’s
concept and chelation theory explains the increased antimicrobial effect as the chelation have
a tendency to make the ligand a more powerful and potent bacterial agent [55]. On chelation,
the polarity of the metal ion will be reduced to a better range due to the overlap of the ligand
orbital and partial sharing of the positive charge of the metal ion with donor groups. Further,
it increases the delocalization of p-electrons over the whole chelate ring and enhances the
penetration of the complexes into lipid membranes and blocking of the metal binding sites in
the enzymes of microorganisms. These complexes may also disturb the respiration process of
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the cell and thus block the synthesis of proteins, which restricts further growth of the
organism [38,56]. In general metal complexes are more active than the ligands because metal
complexes may serve as a vehicle for activation of ligands as the principle cytotoxic species.
4.10. Antioxidant activity
The antioxidant activity of the compounds is evaluated by using the scavenging ability of the
stable DPPH (2, 2׳-diphenyl-1-picryl hydrazyl) radical. The scavenging capacity of DPPH•
was determined spectrophotometrically. DPPH (0.004%) was prepared in methanol and 1 ml
of 0.004% DPPH methanol solution was mixed with 2 ml of (2 mg/ml) methanol compound
mixture. The reaction tubes were wrapped in aluminium foil and kept in dark for 30 min. The
reduction of the DPPH• was monitored by observing the decrease in absorbance at 517 nm
using UV–Vis spectrophotometer with solvent and DPPH as blank. The percentage inhibition
was calculated using the following formula
ꢀ
ꢁꢂꢃꢄꢅꢂꢆ − ꢀꢇꢈꢉꢊꢆꢋ
( )
Percent % inhibition of DPPH activity =
× 100
ꢀꢁꢂꢃꢄꢅꢂꢆ
Where Acontrol = absorbance of DPPH• in methanol without an antioxidant, Asample
absorbance of DPPH• in the presence of an antioxidant.
=
The percentage antioxidant activity of the ligand and its metal complexes are given in
Table 5. The antioxidant activity of synthesized compounds was determined in terms of
either radical scavenging capacity against stable radical DPPH• or hydrogen donation. The
change in color of the DPPH from purple to yellow after reduction, which can be quantified
by its decrease of absorbance at wavelength 517 nm using UV–Vis spectrophotometer. From
the experimental results, the enhanced antioxidant activity is observed for all the metal
complexes than the free ligand. (Figure. 6)
Among all the metal complexes,
[Cu(C32H24N8O4)Cl2] and [Ni(C32H24N8O4)Cl2] showed more antioxidant activity. The values
imply that the activity of the compounds follows the order [Cu(C32H24N8O4)Cl2] >
[Co(C32H24N8O4)Cl2] > [Ni(C32H24N8O4)Cl2] > C32H24N8O4 (L).
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Table 5. Antioxidant activity of Schiff base and its metal complexes
Compound
Antioxidant activity (%)
C32H24N8O4 (L)
23.7
28.5
38.4
33.1
[Ni(C32H24N8O4)Cl2]
[Cu(C32H24N8O4)Cl2]
[Co(C32H24N8O4)Cl2]
M-3
M-2
M-1
(L)
22
24
26
28
30
32
34
36
38
40
% of Antioxidant
Figure 6. Antioxidant activity of macrocyclic ligand and its metal complexes
5. Experimental protocols
All the reagents were of the best grade available and used without further purification. Metal
salts (Merck), O-phthalaldehyde (Merck), diethyl terephthalate and hydrazinhydrate (Aldrich)
were used for the synthesis. All necessary precautions were taken to exclude moisture during
the synthesis and handling of the compounds. Fourier transform infrared (FTIR) spectra of
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the ligand and its metal complexes were recorded in the spectral range 4000-400 cm−1(using
KBr) with a Perkin-Elmer Series 2400 apparatus and FT-IR/FIR Perkin Elmer (Frontier)
1
spectrometer in the range 700-30 cm−1(using CsI) respectively. The H NMR spectra were
recorded on a Varian 400 MHz spectrometer in DMSO against tetramethylsilane (TMS) as
internal reference. Electronic spectra were recorded on a Perkin-Elmer spectrophotometer.
The microanalysis of C, H and N were estimated by Perkin-Elmer 2400 CHNSO elemental
analyser. X-ray diffraction of the synthesised compounds were carried out in powder form
using RIGAKU ULTIMA IV diffractometer system at 25˚C using characteristic Cu
monochromatic radiation and the generator settings as 30 mA, 40 kV. Magnetic susceptibility
measurements were done using Gouy balance.
6. Conclusion
Macrocyclic hydrazide ligand based Ni(II), Cu(II) and Co(II) metal complexes were
synthesized. The physical and analytical data revealed a single metal ion mononuclear six
coordinated octahedral geometry of the complexes, with general formula [M(C32H24N8O4)
Cl2]. Antimicrobial and antioxidant activity profile of the tested compounds revealed that the
metal complexes show enhanced biological activity compared to the free ligand, which could
be attributed to the presence of metal ions in the coordination sphere. Any speculation about
the enhanced biological activity and possible mechanism of action of these complexes,
however, warrent further studies.
Supporting Information
All additional information pertaining to the synthesised compounds is available in
Supplementary Information.
Acknowledgment
The authors are grateful to the University Grants Commission (UGC), New Delhi for
providing UGC-BSR Meritorious research fellowship.
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H I G H L I G H T S
•
•
•
Three new macrocyclic transition metal complexes of Ni2+, Cu2+ and Co2+ were
synthesized.
Synthesized compounds were characterized by various physical and spectroscopic
techniques.
IR spectrum of the ligand indicates the coordination of the azomethine nitrogen
atoms.
•
•
An octahedral geometry has been assigned for Ni2+, Cu2+ and Co2+ complexes.
Antimicrobial activities of the ligand and its complexes, as growth inhibiting agents,
have been screened in vitro against different species of bacteria and fungi.
The complexation led to a remarkable increase in antimicrobial activity.
•