Synthesis, structural characterization, and antibacterial studies of a tetradentate macrocyclic ligand and Its Co(II), Ni(II), and Cu(II) complexes

Article abstract of DOI:10.1134/S1070328409010060

A novel macrocyclic tetradentate ligand 1,5,8,12-tetraaza-2,4,9,11- tetraphenyl-6,7:13,14-dibenzocyclohexadeca- 1,4,8,11-tetraene (L) has been synthesized. Cobalt(II), nickel(II), and copper(II) complexes of this ligand have been prepared and characterize

Full text of DOI:10.1134/S1070328409010060

ISSN 1070-3284, Russian Journal of Coordination Chemistry, 2009, Vol. 35, No. 1, pp. 25–29. © Pleiades Publishing, Ltd., 2009.
Synthesis, Structural Characterization, and Antibacterial Studies
of a Tetradentate Macrocyclic Ligand and Its Co(II), Ni(II),
and Cu(II) Complexes1
S. Chandra*, A. Gautam, and M. Tyagi
Department of Chemistry, Zakir Husain College (University of Delhi), JLN-Marg, New Delhi, 110002 India
* E-mail: schandra_00@yahoo.com
Received January 28, 2008
Abstract—A novel macrocyclic tetradentate ligand 1,5,8,12-tetraaza-2,4,9,11-tetraphenyl-6,7:13,14-dibenzo-
cyclohexadeca-1,4,8,11-tetraene (L) has been synthesized. Cobalt(II), nickel(II), and copper(II) complexes of
this ligand have been prepared and characterized by elemental analysis, molar conductance measurements,
magnetic susceptibitity measurements, and mass, IR, electronic, and ESR spectral studies. The molar conduc-
tance measurements correspond to a nonelectrolytic nature for all the complexes, which can be formulated as
[M(L)X2] (where M = Co(II), Ni(II), and Cu(II); X = Cl– and NO–3 ). On the basis of IR, electronic, and ESR
spectral studies, an octahedral geometry has been assigned to the Co(II) and Ni(II) complexes, whereas a tet-
ragonal geometry was found for the Cu(II) complexes. The investigated compounds and uncomplexed metal
salts and the ligands were tested against bacterial species like Sarcina lutea, Escherchia coli, and Staphylococ-
cus aureus. The metal complexes have higher activity than the free ligand and metal salts.
DOI: 10.1134/S1070328409010060
1
INTRODUCTION
tetraphenyl-6,7:13,14-dibenzocyclohexadeca-1,4,8,11-
tetraene (L) and its Co(II), Ni(II), and Cu(II) com-
plexes.
The synthesis of macrocyclic ligands and their
metal complexes is a growing area of research in inor-
ganic and bioinorganic chemistry in view of their pres-
ence in many biologically significant systems [1–3].
The synthesis and study of macrocycles underwent tre-
mendous growth in recent years, and their complex-
ation chemistry with a wide variety of metal ions has
been extensively studied. Macrocyclic metal com-
plexes are considered to be the model of metallopor-
phyrins [4] owing to their intrinsic structural properties.
Macrocyclic ligands have been used successfully for
diverse processes, such as separation of ions by trans-
port through artificial and natural membranes, liquid–
liquid or solid–solid phase transfer reactions, prepara-
tion of ion-selective electrodes, and isotope separation,
and in the understanding of some natural processes,
through mimicry of metalloenzymes [5]. Macrocyclic
ligands and their metal complexes have a wide range of
biological properties. A lot of work has been done on
transition metal complexes with macrocyclic ligands,
such as antifungal, antibacterial, anticancer, antiviral,
and anti-HIV activity [6–13].
EXPERIMENTAL
All the chemicals used were of AnalaR grade and
procured from Sigma-Aldrich. Metal salts were pur-
chased from E. Merck and were used as received. All
solvents used were of standard (spectroscopic) grade.
Synthesis of the ligand. A hot ethanolic solution
(20 ml) of dibenzoylmethane (4.48 g, 0.02 mol) and a hot
ethanolic solution (20 ml) of o-phenylenediamine (2.16 g,
0.02 mol) were mixed slowly with constant stirring. This
mixture was refluxed at 80°C for 8 h in the presence of
few drops of concentrated hydrochloric acid. On cooling,
a solid light brown precipitate formed, which was filtered
off, washed with cold EtOH, and dried under vacuum over
P4O10. The yield was 68%, mp = 180°C.
For C42H32N4
anal. Òalcd, %:
Found, %:
C, 85.13;
C, 85.20;
H, 5.40;
H, 5.35;
N, 9.45.
N, 9.38.
The present paper describes the synthesis, charac-
terization, and antimicrobial activities of a novel mac-
rocyclic tetradentate ligand, 1,5,8,12-tetraaza-2,4,9,11-
The preparation and structure of the ligand L are
shown below:
1
The text was submitted by the authors in English.
25

26
CHANDRA et al.
C
C
O
O
C
C
N
N
N
N
C
C
Reflux,
+
conc. HCl
H2N
NH2
(L)
Synthesis of the complexes. A hot ethanolic (20 ml) ence of a sharp multiplet in the region δ 7.27 ppm can
solution of the ligand (0.592 g, 0.001 mol) and a hot be assigned to the benzoid hydrogen (6H), having the
ethanolic (20 ml) solution of the corresponding metal same chemical shift and thus behaving as equivalent
salt (0.001 mol) were mixed together with constant stir- [14]. A signal at δ 1.82–2.23 ppm can be assigned to
ring. The mixture was refluxed for 5–7 h at 75–80°C. (C−CH2–C, 4H), and a strong triplet appeared for
On cooling, a colored complex was formed, which was (C−C6H5, 20H) at δ 7.93–8.30 ppm.
filtered off, washed with cold EtOH, and dried under
In the IR spectrum of the ligand, the absence of a
vacuum over P4O10.
band in a region of ~3400 cm–1 corresponding to a free
primary diamine and keto group suggests the complete
condensation of the amino group with the keto group.
The appearance of a new strong absorption band at
1592 cm–1 attributable to the characteristic stretching
frequencies of the imino linkage ν(C=N) provides
strong evidence for the presence of a cyclic product [15,
16]. The bands at 755 and 1458 cm–1 are due to the
presence of the phenyl ring in the macrocycle. The
shifting of the ν(C=N) band toward the lower side in the
complexes indicates that coordination takes place
through the nitrogen of the C=N group, thus implying
that the ligand acts in a tetradentate manner.
Physical measurements. Analytical data were
obtained with a Carlo-Erba 1106 elemental analyzer.
Molar conductance was measured on an ELICO
(CM82T) conductivity bridge. Magnetic susceptibility
was measured at room temperature on a Gouy balance
using CuSO4 · 5H2O as a callibrant. Electron impact
mass spectra were recorded on TOF MS ES + mass
spectrometer. 1H NMR spectra was recorded on a Hita-
chi FT-NMR (model R–600) spectrometer using CDCl3
as a solvent. Chemical shifts are given in ppm relative
to tetramethylsilane. IR spectra (KBr pellets) were
recorded on a FT-IR Spectrum BX-II spectrophotome-
ter. The electronic spectra were recorded in DMF on a
Shimadzu UV mini-1240 spectrophotometer. ESR
spectra of the complexes were recorded as polycrystal-
line samples and in a DMSO solution at liquid nitrogen
temperature (LNT) for the Co(II) complex and at room
temperature (RT) for the Cu(II) complex on an E4-EPR
spectrometer using DPPH as the g-marker. Cyclic vol-
tammetry of the complexes was recorded in DMF at a
scan rate of 100 mV s–1. The redox potential was
recorded with Ag/AgCl as the reference electrode and
platinum as the working electrode, tetrabutylammo-
nium phosphate was the supporting electrolyte, and the
concentration of the complex was 1 × 10–3 mol/l.
On the basis of elemental analysis data, the com-
plexes were found to have the composition as shown in
Table 1. The molar conductance measurements of the
complexes in DMSO correspond to a nonelectrolyte
nature. Thus, these complexes may be formulated as
[M(L)X2] (where M = Co(II), Ni(II), and Cu(II); X =
Cl– and NO–3 ). The IR spectra of the complexes show
bands toward the lower side, indicating coordination
through the nitrogen atom of the C=N groups. The pres-
ence of the absorption bands at 1473–1480, 1309–
1315, and 1000–1014 cm–1 in the IR spectra of the
Co(II), Ni(II), and Cu(II) of the nitrato complexes sug-
gests that both nitrate groups are coordinated to the cen-
tral metal ion in a unidentate manner [17].
RESULTS AND DISCUSSION
Cobalt(II) complexes. At room temperature, the
magnetic moment of the Co(II) complexes lies in the
range 4.91–5.04 µB corresponding to three unpaired
electrons (Table 2). The electronic spectra of the Co(II)
complexes give three peaks in the range 9619–9950 (ε =
68.5 l), 14862–15503 (ε = 75), and 18621–19493 cm–1
(ε = 92 l mol–1 cm–1). These bands may be assigned to
The electron impact mass spectrum of the metal-
free ligand confirms the proposed formula by showing
a peak at 616 amu (i.e., atomic mass 592 corresponding
to the macrocyclic moiety [C42H32N4] + atomic mass 23
of Na+ ion). The spectrum shows a series of peaks, i.e.,
300, 326, 367, 466, 480, 524, 582, etc., corresponding
1
to various fragments. The H NMR spectrum of the
4
4
the transitions T1g(F)
4T2g(F) (ν1), T1g
4A2g
ligand L in CDCl3 does not give any signal correspond-
ing to primary diamine or alcoholic protons. The pres-
4
(ν2), and T1g (F)
4T1g(P) (ν3), respectively. The
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SYNTHESIS, STRUCTURAL CHARACTERIZATION, AND ANTIBACTERIAL STUDIES
27
Table 1. Elemental analysis data and some physical properties of the complexes
Contents (found/calcd), %
Molarconduc-
tance,
Melting- Yield,
Complex
Color
point,°C
%
Ω–1 cm2 mol–1
C
H
N
M
[Co(L)Cl2], CoC42H32N4Cl2
[Co(L)(NO3)2], CoC42H32N6O6
[Ni(L)Cl2], NiC42H32N4Cl2
12
08
15
Pink
Pink
275
282
290
60
58
65
69.87/69.80 4.51/4.43 7.84/7.75 8.23/8.15
65.12/65.03 4.21/4.12 10.76/10.83 7.67/7.60
69.74/69.80 4.38/4.43 7.68/7.75 8.22/8.13
Light
green
[Ni(L)(NO3)2], NiC42H32N6O6
10
Light
green
284
60
65.12/65.03 4.19/4.12 10.90/10.83 7.49/7.57
[Cu(L)Cl2], CuC42H32N4Cl2
09
13
Green
Green
287
278
65
62
69.41/69.32 4.32/4.40 7.63/7.70 8.81/8.73
64.55/64.61 4.14/4.10 10.66/10.76 8.22/8.14
[Cu(L)(NO3)2], CuC42H32N6O6
positions of these bands suggest an octahedral environ- The complexes can be considered to possess a tetrago-
ment around the Co2+ ions [18].
nal geometry.
The ESR spectra of the Co(II) complexes were
recorded as polycrystalline samples and in a DMSO
solution at LNT. The g values were found to be almost
the same in both cases in polycrystalline samples, as
well as in the solution (Table 3). The large deviation in
the g values in the ESR spectra from the spin only value
(g = 2.0023) is due to a large angular momentum con-
tribution [19]. This indicates that the complexes have
the same geometry in the solid, as well as in solution.
The ESR spectra of the Cu(II) complexes were
recorded at room temperature as polycrystalline sam-
ples and in a DMSO solution at LNT. The polycrystal-
line spectra show a well-resolved anisotropic broad sig-
nal [22]. The analysis of the spectra give g|| = 2.22–2.28
and g⊥ = 2.05–2.10 (Table 3). The trend g|| > g⊥ > 2.0023
observed for the complexes indicates that the unpaired
electron is localized in the dx
orbital of the cop-
2 – y2
per(II) ion, and the spectral figures are characteristic of
the axial symmetry. The tetragonal elongated structure
is thus confirmed for the aforesaid complexes [23].
Nickel(II) complexes. The magnetic moment of the
Ni(II) complexes at room temperature lies in the range
2.98–3.14 µB (Table 2). These values are in tune with a
high spin configuration and show the presence of an
octahedral environment around the Ni2+ ion in the com-
plexes. The electronic spectra of the Ni(II) complexes
show bands in the range 9898–10449 (ε = 42.5),
15037–17600 (ε = 67), and 22 675–24 189 cm–1
(ε = 96.5 l mol–1 cm–1) characteristic of an octahedral
geometry [20] and presumably possessing D4h symme-
try. Thus, these bands can be assigned to the three spin-
Various ligand field parameters were calculated for
the complexes and are listed in Table 4. The Nephelaux-
etic parameter β was readily obtained by using the rela-
Table 2. Magnetic moment and electronic spectral data of
the complexes
Complex
µeff, µB
Electronic spectral bands, cm–1
allowed transitions 3A2g(F)
3T1g(F) (ν2), and 3A2g(F)
3T2g(F) (ν1), 3A2g(F)
3T1g(P) (ν3), respectively.
[Co(L)Cl2]
5.04
4.91
2.98
3.14
2.11
1.95
9950, 15503, 18621
9619, 14862, 19493
10449, 15037, 22675
9898, 17600, 24189
10427, 18621, 25773
10438, 18621, 28169
Copper(II) complexes. The magnetic moment values
for the Cu(II) complexes lie in the range 1.95–2.11 µB
corresponding to one unpaired electron. The complexes
may be considered to possess a tetragonal geometry.
The electronic spectra of the Cu(II) complexes reported
here display bands in the range 10427–10438 (ε =
57.5), 18621 (ε = 65), and 25773–28169 cm–1 (ε =
154.5 l mol–1 cm–1). The first two bands can be assigned
[Co(L)(NO3)2]
[Ni(L)Cl2]
[Ni(L)(NO3)2]
[Cu(L)Cl2]
to the transitions 2B1g
2B1g 2B2g (dx
the third band is due to charge-transfer spectra [21].
2A1g (dx
dz2 ) (ν1) and
2 – y2
dzy ) (ν2), respectively, and
2 – y2
[Cu(L)(NO3)2]
RUSSIAN JOURNAL OF COORDINATION CHEMISTRY Vol. 35 No. 1 2009

28
CHANDRA et al.
Table 4. Ligand field parameters of the complexes
Table 3. ESR spectral data of the complexes
Data as
Data in DMSO
solution
Complex
Dq, cm–1 B, cm–1
β
LFSE, kJ mol–1*
Tem-
pera-
ture
polycrystalline
Complex
[Co(L)Cl2]
1243
1203
1044
989
672
650
0.60
0.58
118.94
115.11
149.85
141.96
g||
g⊥ giso g||
g⊥ giso
[Co(L)(NO3)2]
[Ni[(L)Cl2]
[Co(L)Cl2]
LNT 3.79 3.35 3.49 3.95 3.55 3.68
1023 0.98
970 0.93
[Co(L)(NO3)2] LNT 3.52 3.20 3.31 3.70 3.49 3.56
[Cu(L)Cl2] RT 2.28 2.10 2.16 2.25 2.07 2.13
[Ni(L)(NO3)2]
[Cu(L)(NO3)2] RT 2.26 2.06 2.12 2.22 2.05 2.11
* LFSE is ligand field stabilization energy.
tion β = B(complex)/B(free ion), where B(free ion) for
Co(II) is 1120 cm–1 and for Ni(II) is 1041 cm–1 [24]. The
values of β lie in a range of 0.58–0.98. These values
indicate the appreciable covalent character of the metal
ligand σ bond. On the basis of the above spectral stud-
ies, the following structure can be suggested for the
complexes. The structure of the complexes is shown
below:
The antibacterial activities of the ligand, metal salts,
and the complexes have been tested for their effects on
the growth of microbial cultures and were studied for
their interaction with Sarcina lutea, Escherchia coli, and
Staphylococcus aureus using the disc diffusion method
[25, 26]. The test compounds in measured quantities
were dissolved in DMF to get concentrations of 125,
250, and 500 ppm of the compounds. Twenty-five mil-
liliters of nutrient agar media were poured in each Petri
plate. After solidification, 0.1 ml of test bacteria was
spread over the medium using a spreader. The discs of
Whatmann no. 1 filter paper were placed at four equi-
distant places at a distance of 2 cm from the center in
the inoculated Petri plates. Filter paper discs treated
with DMF served as a control and Streptomycin was
used as a standard drug. All determinations were made
in duplicate for each of the compounds. Finally, the
Petri plates were incubated for 26–30 h at 28 2°C. The
zone of inhibition was calculated in millimeters care-
fully.
X
M
X
C N
C N
N
N
C
C
The antimicrobial screening data (Table 5) show
that metal chelates exhibit higher inhibitory effects than
the parents ligand and metal salts. The increased activ-
ity of the metal chelates can be explained on the basis
of chelation theory [27]. It is known that chelation
tends to make the ligand act as a more powerful and
potent bactericidal agent, thus killing more of the bac-
teria than the ligand. It is observed that, in the complex,
the positive charge of the metal is partially shared with
the donor atoms present in the ligands and there is may
be π-electron delocalization over the whole chelating.
This increases the lipophilic character of the metal che-
late and favors its permeation through the lipoid layer
of the bacterial membranes. The bacterial growth inhib-
itory capacity of the ligands, metal salts, and metal
complexes shows the following order: Cu(II) > Ni(II) >
Co(II) > metal salts > ligand.
–
–
M = Co(II), Ni(II), and Cu(II); X = Cl and NO3
The cyclic voltammogram of the copper(II) com-
plexes shows a quasi-reversible peak. The chloro com-
plex gives a peak at 0.70 V, which corresponds to
Cu(III), and a cathodic peak at Epc = 0.37 V,
Cu(II), while the
nitrato complex gives peaks at Epc = 0.68 and 0.40 V.
Cu(II)
which corresponds to Cu(III)
The complexes exhibit irreversible peaks characteristic
for the Cu(II)
Cu(I)
Cu(I) (Epc = –0.54 and –0.69 V) and
Cu(0) (Epc = –0.90 and –0.93 V). The peaks
suggest single-electron reduction processes. On the
anodic side, two peaks are also observed, which corre-
spond to the oxidation Cu(0)
Cu(II). This quasi-
reversible behavior of the Cu(II)/Cu(III) couple was
observed by varying the scan rates with peak potentials.
It has also been observed that Epc increases with
increasing scan rate. Ipc/Ipa (where Ipc = cathodic peak
current and Ipa = anodic peak current) being equal to
unity indicates the chemical reversibility of the redox
process.
AKNOWLEDGMENTS
We are grateful to the UGC (New Delhi) for finan-
cial support; SAIF, IIT (Mumbai), for recording ESR
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SYNTHESIS, STRUCTURAL CHARACTERIZATION, AND ANTIBACTERIAL STUDIES
29
Table 5. Antibacterial screening data of the ligand and its complexes
Diameter of Inhibition Zone, mm (c, µg ml–1)
Compound
Sarcina lutea
Echerchia coli
Staphylococcus aureus
125
250
500
125
250
500
125
250
500
L
09
14
13
12
11
14
14
15
14
17
17
20
22
24
14
17
15
16
16
19
18
20
20
22
24
25
27
28
19
22
21
25
24
26
25
28
27
29
30
31
32
34
10
12
12
11
11
12
12
14
16
16
14
19
18
20
15
19
18
17
17
18
17
20
19
22
24
29
27
30
21
23
23
22
23
24
24
25
27
31
31
32
34
36
00
08
09
08
08
10
09
19
20
21
22
25
24
26
12
16
14
13
12
16
16
22
22
25
29
32
31
32
17
20
21
19
18
22
22
31
29
35
34
37
29
40
CoCl2 · 6H2O
Co(NO3)2 · 3H2O
NiCl2 · 6H2O
Ni(NO3)2 · 5H2O
CuCl2 · 5H2O
Cu(NO3)2 · 3H2O
[Co(L)Cl2]
[Co(L)(NO3)2]
[Ni[(L)Cl2]
[Ni(L)(NO3)2]
[Cu(L)Cl2]
[Cu(L)(NO3)2]
Streptomycin
13. Parkinson, J.A., Weishaupl, M., Gould, R.O., et al., J.
Am. Chem. Soc., 2002, vol. 124, p. 9105.
spectra; and ACBR (New Delhi) for recording IR
spectra.
14. Kalsi, P.S., Spectroscopy of Organic Compounds, New
Delhi (India): New Age International (P) Ltd., 1999.
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