New 14-membered trans-di-substituted 'tet-a' macrocycles and their copper(II) and nickel(II) complexes: Spectral, magnetic, electrochemical, crystal structure, catalytic and antimicrobial studies

Article abstract of DOI:10.1016/j.molstruc.2011.01.010

Copper(II) and nickel(II) complexes of new trans-di-substituted macrocyclic ligands incorporating three different benzoyl groups, 1,8-bis-(benzoyl)-5,5,7, 12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (L¹), 1,8-bis-(2-nitrobenzoyl)-5,5,7,12,12,14-hexamethyl-1,4,8,11- tetraazacyclotetradecane (L²), and 1,8-bis-(4-nitrobenzoyl)-5,5,7,12, 12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (L³) have been synthesized. The ligands and complexes were characterized by elemental and spectral analysis. Further, the X-ray crystal structure of one of the ligand, L¹, has been determined. N-benzoylation causes red shift in the λ_max values of the complexes. The cyclic voltammogram of the complexes of ligand L¹ shows one-electron, quasi-reversible reduction wave in the region -0.94 to -1.00 V, whereas that of L² and L ³ show two quasi-reversible reduction peaks corresponding to ligand and metal. Nickel complexes show one electron quasi-reversible oxidation wave at a positive potential in the range +0.92 to +1.21 V. The Cu(II) complexes have a magnetic moment value close to the spin-only value with four hyperfine EPR signals. Kinetic studies on the oxidation of pyrocatechol to o-quinone using the copper(II) complexes as catalysts and on the hydrolysis of 4- nitrophenylphosphate using the copper(II) and nickel(II) complexes as catalysts were carried out. The complexes containing electron-withdrawing group shows higher rate-constant values than that with no electron-withdrawing group. All the complexes were screened for antifungal and antibacterial activity.

Full text of DOI:10.1016/j.molstruc.2011.01.010

Journal of Molecular Structure 989 (2011) 91–100
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
New 14-membered trans-di-substituted ‘tet-a’ macrocycles and their copper(II)
and nickel(II) complexes: Spectral, magnetic, electrochemical, crystal structure,
catalytic and antimicrobial studies
G. Nirmala a, A. Kalilur Rahiman b, S. Sreedaran c, R. Jegadeesh d, N. Raaman d, V. Narayanan a,
⇑
a Department of Inorganic Chemistry, School of Chemical Sciences, University of Madras, Guindy Campus, Chennai 600 025, India
b PG & Research Department of Chemistry, The New College (Autonomous), Chennai 600 014, India
c Department of Chemistry, Government Arts College, Udhagamandalam 643 002, India
d Centre for Advanced Studies in Botany, University of Madras, Guindy Campus, Chennai 600 025, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Copper(II) and nickel(II) complexes of new trans-di-substituted macrocyclic ligands incorporating three
different benzoyl groups, 1,8-bis-(benzoyl)-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradec-
ane (L1), 1,8-bis-(2-nitrobenzoyl)-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (L2),
and 1,8-bis-(4-nitrobenzoyl)-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (L3) have
been synthesized. The ligands and complexes were characterized by elemental and spectral analysis. Fur-
ther, the X-ray crystal structure of one of the ligand, L1, has been determined. N-benzoylation causes red
shift in the kmax values of the complexes. The cyclic voltammogram of the complexes of ligand L1 shows
one-electron, quasi-reversible reduction wave in the region ꢀ0.94 to ꢀ1.00 V, whereas that of L2 and L3
show two quasi-reversible reduction peaks corresponding to ligand and metal. Nickel complexes show
one electron quasi-reversible oxidation wave at a positive potential in the range +0.92 to +1.21 V. The
Cu(II) complexes have a magnetic moment value close to the spin-only value with four hyperfine EPR sig-
nals. Kinetic studies on the oxidation of pyrocatechol to o-quinone using the copper(II) complexes as cat-
alysts and on the hydrolysis of 4-nitrophenylphosphate using the copper(II) and nickel(II) complexes as
catalysts were carried out. The complexes containing electron-withdrawing group shows higher rate-
constant values than that with no electron-withdrawing group. All the complexes were screened for anti-
fungal and antibacterial activity.
Received 20 October 2010
Received in revised form 6 January 2011
Accepted 7 January 2011
Available online 22 January 2011
Keywords:
Tet-a
Benzoylation
Catecholase activity
Hydrolysis of 4-nitrophenylphosphate
Antimicrobial activity
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
attention largely due to their structures and properties which are
different from their unsubstituted parent macrocycles [6].
The field of coordination chemistry of tetraaza macrocycles has
been studied extensively [1–3]. One of the most popular tetraaza
macrocyclic ligands used for decades in coordination chemistry is
1,4,8,11-tetraazatetradecane, universally known as cyclam [4]. For-
mation of cyclam-type tetraaza macrocycle was first reported by
Curtis [5], who performed condensation of two molecules of
ethylenediamine with four molecules of acetone in presence of
nickel(II) or copper(II) to obtain metal complexes of 5,7,7,12,14,14-
hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene, which
can be regarded as a precursor of hexamethyl substituted cyclam,
namely the ‘tet-a’ (meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetra-
azacyclotetradecane). However, tetraaza macrocyclic complexes
bearing pendant coordination side arms continue to attract
Understanding of N-substitution of the ligand on coordination
remains an important field of research. In order to synthesize the
requisite type of macrocycles, the obvious approach is to first syn-
thesize the simple macrocyclic ligand, and then to attach suitable
pendent hydrocarbon moieties to the periphery of these macrocy-
cles through appropriate reactions [7]. Pendant arms can enhance
the selectivity of the ligand for a given ion and may allow for a fine
tuning the properties of the complex.
The coordination behaviors of the partially N-functionalized
tetraaza macrocycles and their metal complexes have been studied
less extensively than those of the fully N-functionalized derivatives
[8–10]. This may be due to the fact that the synthesis of the
partially N-functionalized macrocycle is somewhat complicated
and requires several steps. It has been well known that the
chemical properties and structures of the partially and fully
N-functionalized macrocyclic compounds are distinctly different
from those of the unsubstituted ones.
⇑
Corresponding author. Tel./fax: +91 44 2230 0488.
E-mail address: vnnara@yahoo.co.in (V. Narayanan).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.01.010

92
G. Nirmala et al. / Journal of Molecular Structure 989 (2011) 91–100
In this work, we describe the synthesis of three new trans func-
standard and E1/2 of the ferrocene/ferrocenium (Fc/Fc+) couple un-
der the experimental condition was 470 mV. Tetra(n-butyl)ammo-
nium perchlorate (TBAP) was used as the supporting electrolyte.
Room temperature magnetic moments were measured on a PAR
vibrating sample magnetometer Model-155. X-band EPR spectra
were recorded at 25 °C on a Varian EPR-E 112 spectrometer using
diphenylpicrylhydrazine (DPPH) as the reference. Catalytic oxida-
tion of catechol to o-quinone by the copper complexes and hydro-
lysis of 4-nitrophenylphosphate by the copper and nickel
complexes were studied in 10ꢀ3 M dimethylformamide solutions.
The catechol oxidation reactions were followed spectrophotomet-
rically by choosing the strongest absorbance at 390 nm
(o-quinone) and the phosphate hydrolysis reactions were followed
at 420 nm (4-nitrophenolate anion). The increase in the absor-
bance at these wavelengths was monitored as a function of time.
tionalized hexamethyl tetraazacyclotetradecane macrocycles, 1,8-
bis-(benzoyl)-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclo-
tetradecane (L1), 1,8-bis-(2-nitrobenzoyl)-5,5,7,12,12,14-hexa-
methyl-1,4,8,11-tetraazacyclotetradecane
(L2),
1,8-bis-(4-
nitrobenzoyl)-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclo-
tetradecane (L3). The effects of the N-pendant groups on their
chemical and coordination behaviors have been investigated.
2. Experimental
2.1. Analytical and physical measurements
Elemental analyses were carried out using a Haereus CHN rapid
analyzer. IR spectra were recorded on a Shimadzu FT-IR 8300 series
spectrophotometer in the range 4000–400 cmꢀ1 using KBr disks.
1H NMR spectra were recorded using a JEOL GSX 400 MHz NMR
spectrometer. The atomic absorption spectral data were recorded
in DMSO using a Varian spectra AA-200 model atomic absorption
spectrophotometer. FAB+ mass spectrum was obtained on a JEOL
SX102/DA-6000 Mass Spectrometer using m-nitro benzyl alcohol
(NBA) as the matrix. Electronic spectral studies were carried out
on a Hitachi 320 spectrophotometer in the range 200–1100 nm.
Molar conductivity was measured by using an Elico digital conduc-
tivity bridge model CM-88 using freshly prepared solution of the
complex in dimethylformamide. Cyclic voltammograms were ob-
tained on a CHI-600A electrochemical analyzer. The measurements
were carried out under oxygen-free conditions using a three-
electrode cell in which a glassy carbon electrode was the working
A plot of log (A /A1 ꢀ At) versus time was made for each complex
1
and the rate constants for the catalytic oxidations and hydrolysis of
4-nitrophenylphosphate were calculated.
The crystal structure of ligand L1 was measured on a Bruker
SMART APEX CCD area detector diffractometer with graphite
monochromated Mo K
a radiation. The data covered over a hemi-
sphere of reciprocal space by a combination of three sets of expo-
sures; each set had a different ø angle (0°, 88°, and 180°) for the
crystal and each exposure of 10 s covered 0.3° in
x. The detector
was at a distance of 4 cm from the crystal and the swing angle of
the detector was ꢀ35°. Cell refinement and data reduction were
carried out using SMART and SAINT programs [11]. At the end of
the data collection, the crystal decay was monitored by repeating
thirty initial frames and analyzing the duplicate reflections.
Absorption correction was neglected because
only.
l
t = 0.065 mmꢀ1
electrode,
a saturated Ag/AgCl electrode was the reference
electrode and platinum wire was used as the auxiliary electrode.
A ferrocene/ferrocenium (1+) couple was used as an internal
The structure was solved by direct methods using the program
SHELXS-97 [12], a package for crystal structure solution using di-
rect methods. All non-hydrogen atoms were refined anisotropically
[12] by full-matrix least-square procedures with the weight
Table 1
2
w ¼ 1=½r2ðFo2Þ þ ð0:2000PÞ ꢁ, where P ¼ ðFo2 þ 2F2oÞ=3. All hydrogen
Crystal data and structure refinement details for ligand L1.
atoms were geometrically fixed and allowed to ride along with cor-
responding non-hydrogen atoms during the refinement. The crys-
tallographic data and structure refinement details for ligand L1
Crystal data
Empirical formula
Formula weight
Temperature (K)
Radiation used
Wavelength (Å)
Crystal system
Space group
C30H44N4O2Cl2
565.50
293
Mo K
a
0.71073
Monoclinic
P21/c
Table 2
Selected bond lengths [Å] and angles [°].
N1AC7
N1AC14
N1AC8
N2AC15
N2AC11
C7AO1
C7AN1AC14
C7AN1AC8
C14AN1AC8
C15AN2AC11
O1AC7AN1
O1AC7AC1
N1AC7AC1
N2AC11AC10
1.357(2)
1.461(2)
1.484(2)
1.494(2)
1.530(2)
1.228(2)
117.5(1)
124.6(1)
117.8(1)
117.9(1)
120.4(2)
119.4(2)
120.2(1)
104.3(1)
Unit cell dimensions
a = 13.3451(7) Å
b = 7.9364(4) Å; b = 99.7(5)°
c = 15.1082(7) Å
Volume (V)
1577.02(14) Å3
Z
2
Dcal (Mg mꢀ3
)
1.2543
0.065
636
Absorption coefficient (mmꢀ1
F(0 0 0)
)
Crystal size (mm)
0.30 ꢂ 0.26 ꢂ 0.20
Data collection
Diffractometer used
hmax range
SMART CCD
33.39°
Limiting indices
h
k
ꢀ17 6 h 6 20
ꢀ12 6 k 6 12
ꢀ23 6 l 6 23
20993
6035
100%
l
Reflection collected
Unique reflections
Completeness to h
Table 3
Hydrogen bonding [Å and °].
Refinement
DAHꢃ ꢃ ꢃA
d (DAH)
d (Hꢃ ꢃ ꢃA)
d (Dꢃ ꢃ ꢃA)
<(DHA)
Refinement method
Data/parameters
Goodness-of-fit on F2
Final R indices [I > 2
Largest diff. peak (e Åꢀ3
Largest diff. hole (e Åꢀ3
Full-matrix least squares on F2
6035/194
1.02
R1 = 0.065, wR2 = 0.222
0.84
ꢀ0.53
N2AH2Dꢃ ꢃ ꢃCl
N2AH2Cꢃ ꢃ ꢃCl
C14AH15Bꢃ ꢃ ꢃCl
C10AH10Bꢃ ꢃ ꢃCl
C10–H10Aꢃ ꢃ ꢃCl
0.83
0.83
0.97
0.97
0.97
2.23
2.38
2.83
2.81
2.63
3.041(1)
3.201(2)
3.701(2)
3.778(2)
3.544(2)
165.0
171.0
150.1
175.6
156.4
r
(I)]
)
)

G. Nirmala et al. / Journal of Molecular Structure 989 (2011) 91–100
93
are given in Table 1. The selected interatomic distances and angles
are given in Table 2 whereas hydrogen bond dimensions in Table 3.
2.4. General procedure for synthesis of macrocyclic copper(II) and
nickel(II) complexes
The copper(II) and nickel(II) complexes were prepared by
refluxing the respective metal perchlorate hexahydrate (1 mmol)
in DMF (50 mL) with appropriate ligands (1 mmol) in DMF
(50 mL) for 8–12 h. The resulting solution was then filtered while
hot and allowed to stand at room temperature. After the evapora-
tion of solvent, the compound was recrystallized from acetonitrile
to get the pure complexes.
2.2. Chemicals and reagents
5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane
(tet-a) was prepared by following the literature method [13].
Benzoyl chloride and nitrobenzoyl chlorides were purchased from
Sigma–Aldrich and used as such. Analytical grade methanol,
acetonitrile and dimethylformamide were purchased from
Qualigens and used as such. TBAP was used as supporting electro-
lyte in electrochemical measurements was purchased from Fluka
and recrystallized from hot methanol. (Caution! TBAP is poten-
tially explosive; hence care should be taken in handling the com-
pound). All other chemicals and solvents were of analytical grade
and were used as received without any further purification.
2.4.1. [CuL1](ClO4)2
Brown solid. Yield:1.07 g (70%). Analytical data: C30H44N4O10
Cl2Cu: Calculated (%): C, 47.71; H, 5.87; N, 7.41, Cu, 8.42. Found:
C, 47.62; H, 5.75; N 7.33, Cu, 8.40. Selected IR: 3180 m(NH), 1636
m
(C@O). FAB mass (m/z (%)): [CuL1(ClO4)AClO4]+: 655. Conduc-
tance (km/S cm2 molꢀ1) in CH3CN: 143. kmax, nm (
e
, Mꢀ1 cmꢀ1) in
DMF: 498 (169) 374 (15,400), 305 (33,600). gll = 2.23, g\ = 2.09
and All = 165. eff = 1.74 BM.
2.3. Synthesis of the ligands
l
2.3.1. Synthesis of 1,8-bis-(benzoyl)-5,5,7,12,12,14-hexamethyl–
1,4,8,11-tetraazacyclotetradecane (L1)
2.4.2. [CuL2](ClO4)2
Two equivalents of benzoyl chloride (0.9 g, 0.007 mol) was
added slowly to 5,5,7,12,12,14 hexamethyl-1,4,8,11-tetraazacyclo-
tetradecane (1 g, 0.0035 mol) dissolved in 50 mL of acetonitrile and
stirred vigorously for 36 h. The crude product obtained was recrys-
tallized from DMF to get pure compound as white solid.
Yield: 1.5 g (80%). M.p: 280 °C. Analytical data for C30H44N4O2:
Calculated (%): C, 73.13; H, 9.00; N, 11.37. Found: C, 73.06; H,
Brown solid. Yield: 0.98 g (80%). Analytical data:
C
30H42N6O14Cl2Cu: Calculated (%): C, 42.63; H, 5.00; N, 9.94, Cu,
7.52. Found: C, 42.53; H, 4.95; N, 9.90, Cu, 7.50. Selected IR: 3182
(NH), 1628 (C@O), 1548, 1345 (NO2). FAB mass (m/z (%)):
[CuL2A2ClO4]+: 646. Conductance (km/S cm2 molꢀ1) in CH3CN:
130. kmax nm (
, Mꢀ1 cmꢀ1) in DMF: 547 (645), 410 (11,285), 309
m
m
m
e
(22,600). gll = 2.21, g\ = 2.09 and All = 175. leff = 1.73 BM.
8.98; N, 11.27. Selected IR (KBr)(
m
t m(NH), 1654
/cmꢀ1): 3185
(C@O). 1H NMR (d ppm in C2D6SO): 1.18 (m, 12H), 1.58 (m, 6H),
1.78 (m, 4H), 2.10 (m, 4H), 2.14 (m, 4H), 7.32 (d, 4H), 7.47 (t,
2H), 7.89(m, 4H). 13C NMR (d ppm in C2D6SO): 20.2, 24.0, 37.4,
39.2, 45.7, 48.1, 55.3, 124.6, 127, 127.5, 129.0, 129.2, 135.4,
2.4.3. [CuL3](ClO4)2
Brown solid. Yield: 0.98 g (80%). Analytical data:
30H42N6O14Cl2Cu: Calculated (%): C, 42.63; H, 5.00; N, 9.94, Cu.
C
168.3. kmax nm (e
, Mꢀ1 cmꢀ1) in DMF: 280 (29, 760).
7.52. Found: C, 42.60; H, 4.92; N, 9.88, Cu, 7.50. Selected IR: 3195
(NH), 1630 (C@O), 1550, 1340 (NO2). FAB mass (m/z (%)):
[CuL3A2ClO4]+: 645. Conductance (km/S cm2 molꢀ1) in CH3CN:137.
kmax nm (
, Mꢀ1 cmꢀ1) in DMF: 520 (127), 438 (10,200), 303
m
m
m
2.3.2. Synthesis of 1,8-bis-(2-nitrobenzoyl)-1,4,8,11-tetraaza-
5,5,7,12,12,14-hexamethyl cyclotetradecane (L2)
e
Ligand L2 was prepared by a method similar to that described
for L1 by using 2-nitrobenzoyl chloride instead of benzoyl chloride.
Recrystallization from DMF gave yellowish white solid.
Yield: 1.3 gm (65%). M.p: 290 °C. Analytical data for
(21,300). gll = 2.24, g\ = 2.10 and All = 160. leff = 1.73 BM.
2.4.4. [Ni L1](ClO4)2
Yellowish green solid. Yield: 0.97 g (65%). Analytical data:
30H44N4O10Cl2Ni: Calculated (%): C, 48.67; H, 5.99; N, 7.56, Ni,
C
30H42N6O6: Calculated (%): C, 61.84: H, 7.26; N, 14.42. Found: C,
61.76: H, 7.20; N, 14.37. Selected IR (KBr)( (NH),
/cmꢀ1): 3190
1660 (C@O), 1550, 1347
(NO2). 1H NMR (d ppm in C2D6SO):
C
t
m
7.82. Found: C, 48.58; H, 5.90; N, 7.46, Ni, 7.90. Selected IR: 3183
m
m
m
(NH), 1635 m
(C@O). FAB mass (m/z (%)): [NiL1A2ClO4]+: 551. Con-
1.17 (m, 12H), 1.54 (m, 6H), 1.72 (m, 4H), 1.95 (m, 4H), 2.13 (m,
4H), 7.29 (m, 4H), 8.17 (t, 2H), 8.35 (m, 2H). 13C NMR (d ppm in
C2D6SO): 19.2, 24.3, 37.0, 39.2, 46.2, 48.1, 55.1, 124.6, 127.0,
ductance (km/S cm2 molꢀ1) in CH3CN: 139. kmax nm ( , Mꢀ1 cmꢀ1
e )
in DMF: 482 (137), 410 (14,280), 322 (20,210).
127.2, 129.0, 134.2, 146.4, 168.3. kmax nm (e
, Mꢀ1 cmꢀ1) in DMF:
289 (25, 890).
2.4.5. [NiL2](ClO4)2
Yellowish green solid. Yield: 1.00 g (69%). Analytical data:
30H42N6O14Cl2Ni: Calculated (%): C, 42.88; H, 5.03; N, 10.00, Ni,
2.3.3. Synthesis of 1,8-bis-(4-nitrobenzoyl)-1,4,8,11-tetraaza-
5,5,7,12,12,14-hexamethyl cyclotetradecane (L3)
C
6.98. Found: C, 42.79; H, 4.92; N, 9.89, Ni, 6.96. Selected IR: 3185
Ligand L3 was prepared by a method similar to that described
for L1 by using 4-nitrobenzoyl chloride instead of benzoyl chloride.
Recrystallization from DMF gave yellowish white solid.
Yield: 1.33 g (65%). M.p: 290 °C. Analytical data for C30H42N6O6:
Calculated (%): C, 61.84: H, 7.26; N, 14.42. Found: C, 61.70: H, 7.18;
m
(NH), 1622 (C@O), 1548, 1342
m
m
(NO2). [NiL2(ClO4)AClO4]+: 741.
Conductance (km/S cm2 molꢀ1
) in CH3CN: 137. kmax nm (e,
Mꢀ1 cmꢀ1) in DMF: 527 (168), 415 (10,840), 305 (21,500).
N, 14.35. Selected IR (KBr)(
t
/cmꢀ1): 3198
m
(NH), 1658
m(C@O),
2.4.6. [NiL3](ClO4)2
Yellowish green solid. Yield: 1.00 g (69%). Analytical data:
C30H42N6O14Cl2Ni: Calculated (%): C, 42.88; H, 5.03; N, 10.00, Ni,
1554, 1345
m
(NO2). 1H NMR (d ppm in C2D6SO): (d ppm in
DMSO-D6): 1.15 (m, 12H), 1.56 (m, 6H), 1.74 (m, 4H), 1.92 (m,
4H), 7.25 (d, 4H), 8.09 (t, 2H), 8.24 (d, 2H). 13C NMR (d ppm in
C2D6SO): 19.2, 24.3, 37.0, 39.2, 46.2, 48.1, 55.1, 125.0, 127.0,
6.90. Found: C, 42.79; H, 4.92; N, 9.89, Ni, 6.88. Selected IR: 3196
m
(NH), 1628 m(C@O), 1550, 1345 m
(NO2). [NiL3A2ClO4]+: 642. Con-
127.1, 129.0, 129.6, 147.7, 167.2. kmax nm (
e
, Mꢀ1 cmꢀ1) in DMF:
ductance (km/S cm2 molꢀ1) in CH3CN: 145. kmax nm ( , Mꢀ1 cmꢀ1
e )
290 (25, 940).
in DMF: 545 (278), 423 (13,400), 306 (23,140).

94
G. Nirmala et al. / Journal of Molecular Structure 989 (2011) 91–100
the associated phenyl ring are within the range reported for such
3. Results and discussion
molecule. Bonds to protonated N[N2AC15 = 1.494(2) Å and
N2AC11 = 1.530(2) Å] are significantly longer than bonds to dou-
bly protonated N[N1AC14 = 1.461(2) Å and N1AC8 = 1.484(2) Å].
The sum of the angles about N1 atom (359.9°) is an indication of
sp2 hybridization. Macrocycles such as cyclams and cyclens and
their derivatives can adopt several conformations. In the crystal
structure of the title compound the cation has a conformation close
to centrosymmetric trans-III type with four of the six-methyl
groups in equatorial positions and two in axial positions. The trans
conformation of benzene groups with respect to the cyclic tetra
amines is observed and is similar to that found in N,N0,N00,N000-tetra-
methyl-1,4,8,11-cyclam [17]. The torsion angle about the bond
N1AC7 are notably asymmetric [C14AN1AC7ACl = ꢀ172.5(1)°;
C8AN1AC7AC1 = 11.4(2)°], indicate that the benzene substituent
is not coplanar with the cyclic tetra amines.
Fig. 2 shows the crystal packing of the compound view down ‘a’
axis. The crystal structure of the compound is stabilized by intra-
molecular NAHꢃ ꢃ ꢃCl type hydrogen bond and the crystal packing
is stabilized by the CAHꢃ ꢃ ꢃCl type hydrogen bond. The molecule
at (x, y, z) is linked to its symmetry-related molecule at (1 ꢀ x, 1/
2 + y, 1/2 ꢀ z) and (x, 1/2 ꢀ y, 1/2 + z) through a CAHꢃ ꢃ ꢃCl type
hydrogen bond. Propagation of these linkages generates infinite
molecular chain along the crystallographic ‘c’ axis. In the crystal
packing the adjacent units are not linked by any specific
interaction.
The synthesis of N-substituted ‘‘tet-a’’ ligands (L1–L3) from
5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane and
their metal complexes were shown in Schemes 1 and 2 respec-
tively. The syntheses of these partially N-substituted macrocycles
is generally complicated, but the presence of alkyl substituents
on the macrocyclic ring carbons next to the secondary amino
groups facilitate syntheses, as substitution takes place only on ste-
rically less hindered nitrogen atoms. Even though the nitrogen
atoms at 4 and 11 positions are more basic, they are sterically hin-
dered and only 1,8 functionalization takes place [14,15]. The mag-
netic, electrochemical, catalytic and antibacterial activities of the
complexes are discussed. The molar conductance values of the cop-
per(II) and nickel(II) complexes are fall in the expected range of
130–145 Km/S cmꢀ1 molꢀ1, which indicates that the complexes
are of 1:2 electrolytes [16].
3.1. The structure of ligand L1
The colorless crystals of diffraction quality were obtained by
slow evaporation from a DMSO solution of the ligand. The ORTEP
diagram of the ligand L1 is shown in Fig. 1. The asymmetric unit
contains half a macrocyclic cation and a chloride anion. The macro-
cyclic ligand, 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotet-
radecane exists as two isomers (tet-a and tet-b) and both can bind
in either a planar or a folded geometry, which results in trans or cis
six fold-coordinated complexes. The almost rectangular macrocy-
cle resulting from a combination of increased electrostatic repul-
sion between adjacent cationic N atoms and the lack of any
intramolecular NAHꢃ ꢃ ꢃN hydrogen bond. The four N atoms in the
macrocycle are coplanar and the two symmetry-related N2 atoms
are protonated. Bond distances and angles in the macrocycle and
3.2. Spectral studies
The FT-IR spectra of the ligands L1ꢀ3 show a band in the region
around 3190 cmꢀ1 due to the presence of NAH protons. The bands
in the region of 1654–1660 cmꢀ1 is due to the carbonyl group. The
N-substituted ‘tet-a’ ligands L2&3 show strong absorptions around
O
O
NH
N
N
NH
N
N
NO2
NO2
HN
HN
O
Cl
Cl
O
O
O
NO2
2
L1
L2
2
CH3CN
CH3CN
NH HN
NH HN
Cl
O
2
NO2
O
NO2
NH
N
N
HN
O2N
O
L3
Scheme 1. Synthesis of trans functionalized ‘tet-a’ ligands.

G. Nirmala et al. / Journal of Molecular Structure 989 (2011) 91–100
95
2+
O
O
NH
N
N
NH
N
N
M(ClO4)2.H2O
M
HN
NH
O
O
2+
O
O
NH
N
NH
N
N
NO2
NO2
M(ClO4)2.H2O
M
NO2
NO2
N
HN
NH
O
O
2+
NO2
O
O
NO2
NH
N
N
NH
N
M(ClO4)2.H2O
M
HN
N
NH
O2N
O2N
O
O
M = Cu or Ni
Scheme 2. Synthesis of trans functionalized ‘tet-a’ complexes.
The electronic spectral data are summarized in Table 4. The
electronic spectra of the complexes show one or two peaks in the
range of 303–322 nm and is assigned to the intra-ligand transition
(p–
pꢄ). An intense peak in the range of 374–438 nm is due to li-
gand-to-metal charge transfer transition. The copper(II) complexes
show dAd transition in the region of 498–547 nm, which is typical
for reported square-planar tetraazamacrocyclic copper(II) com-
plexes [21–24]. The electronic spectra of nickel(II) complexes show
a dAd band in the region 482–545 nm, which is consistent with
adoption of
a square-planar geometry with N4 coordination
[25,26]. The observed dAd band for unsubstituted copper(II) and
nickel(II) macrocyclic complexes are 450 and 517 nm, respectively
[27]. The electronic spectra of the complexes show a ‘red shift’ on
N-alkylation. The red shift in the dAd absorption bands of the cop-
per(II) and nickel(II) complexes may be due to tetrahedral distor-
tion of the coordination sphere caused by N-alkylation [28,29].
This may be due to the presence of electron-withdrawing carbonyl
Fig. 1. ORTEP diagram of ligand L1 with atom labels and numbering scheme.
1555 cmꢀ1 and 1345 cmꢀ1 corresponding to ANO2 stretching
vibrations.
and nitro groups in the pendant arms of the ligand L2&3
.
All the complexes show a band in the region 3180–3196 cmꢀ1
due to
m(NAH) stretching indicating that the two secondary amine
3.3. ESR spectral analysis
groups in the tet-a ring are not deprotonated. The infrared spectra
of all the complexes showed the expected shift in the position of
the amide carbonyl band from 1654 to 1660 cmꢀ1 for the free li-
gand to 1622–1636 cmꢀ1 for the complexes, which indicates that
the carbonyl group is not coordinated to the metal ion. The pres-
ence of uncoordinated perchlorate anions in all complexes are in-
The ESR spectrum of mononuclear copper(II) complex [CuL1]
(ClO4)2 is shown Fig. 3 and the ESR spectral data of the complexes
are given in Table 5. The room temperature (25 °C) solid state X-
band ESR spectra of the mononuclear copper(II) complexes show
four lines with nuclear hyperfine spin 3/2. The hyperfine All split-
ting falls in the range of 160–175 ꢂ 10ꢀ4 cmꢀ1, indicative of an
electron interacting with only one copper nucleus. The relation
gll > g\ is typical of d9 copper(II) complexes in a ground state dou-
ferred from single broad band around 1100 cmꢀ1
(m3-
antisymmetric stretching) which are not split and a band around
650 cmꢀ1
930 cmꢀ1
(
m
4-antisymmetric bending). The band around
(m
2-symmetric stretching) corresponding to coordinated
blet with the unpaired electron in a dx2
orbital. The g and All val-
ꢀy2
perchlorate is not observed which indicates that no perchlorate
ions are coordinated in the complexes [18,19]. The mononuclear
complexes of the ligands L2&3 show strong absorptions at
1550 cmꢀ1 and 1345 cmꢀ1 corresponding to ANO2 stretching
vibrations [20].
ues of CuL1ꢀ3 are close to the [Cu(tet-a)]2+ suggesting essentially
square-planar coordination geometry of the copper(II) complexes
[24].
The room temperature (at 25 °C) magnetic moment studies of
the copper(II) complexes show a leff value in the range of

96
G. Nirmala et al. / Journal of Molecular Structure 989 (2011) 91–100
Fig. 3. ESR spectrum of [CuL1](ClO4)2 complex.
Table 5
ESR and magnetic moment data of copper(II) complexes.
Complexes
Magnetic moment leff BM
ESR
gll
g\
All
[CuL1](ClO4)2
[CuL2](ClO4)2
[CuL3](ClO4)2
1.70
1.72
1.74
2.18
2.21
2.24
2.09
2.09
2.10
165
173
160
Table 6
Electrochemical data for the copper(II) and nickel(II) complexes (reduction at
cathodic potential).
E1Pc (V)
E1Pa (V)
E11=2 (V)
E2Pc (V)
S. No
Complexes
D
E1(mV)
1
2
3
4
5
6
[CuL1](ClO4)2
[CuL2](ClO4)2
[CuL3](ClO4)2
[NiL1](ClO4)2
[NiL2](ClO4)2
[NiL3](ClO4)2
ꢀ0.94
ꢀ0.80
ꢀ0.88
ꢀ1.00
ꢀ0.78
ꢀ0.70
ꢀ0.75
ꢀ0.58
ꢀ0.67
ꢀ0.82
ꢀ0.64
ꢀ0.62
ꢀ0.84
ꢀ0.69
ꢀ0.78
ꢀ0.91
ꢀ0.71
ꢀ0.66
190
220
210
180
140
080
ꢀ
ꢀ1.50
ꢀ1.56
ꢀ
Fig. 2. The packing of the molecules of ligand L1, viewed down the ‘a’ axis. dashed
lines indicate hydrogen bond.
ꢀ1.46
ꢀ1.48
Table 4
Electronic spectral data of copper(II) and nickel(II) complexes.
S. No
Compound
[(kmax/nm) (e
, Mꢀ1 cmꢀ1)] in DMF
Table 7
1
2
3
4
5
6
[CuL1](ClO4)2
[CuL2](ClO4)2
[CuL3](ClO4)2
[NiL1](ClO4)2
[NiL2](ClO4)2
[NiL3](ClO4)2
498 (169), 374 (15,400), 305 (33,600)
547 (645), 410 (11,285), 309 (22,600)
520 (127), 438 (10,200), 303 (21,300)
482 (137), 410 (14,280), 322 (20,210)
527 (168), 415 (10,840), 305 (21,500)
545 (278), 423 (13,400), 306 (23,140)
Electrochemical data for the nickel(II) complexes (oxidation at anodic potential).
S. No
Complexes
EPc (V)
EPa (V)
E1/2 (V)
DE (mV)
1
2
3
[NiL1](ClO4)2
[NiL2](ClO4)2
[NiL3](ClO4)2
0.92
1.12
1.21
0.80
0.96
1.03
0.86
1.04
1.12
120
160
180
Figs. 4–6. Cyclic voltammogram for the complexes of the ligand
L1 shows one quasi-reversible reduction wave [32,33] at cathodic
potential of ꢀ0.94 and ꢀ1.00 V. The complexes of the ligands
L2&3 shows both metal and ligand (ANO2) reduction peaks in the
cathodic potential in region of ꢀ0.80 to ꢀ1.30 V and ꢀ1.26 to
ꢀ1.61 V respectively. The first reduction potential is quasi-
reversible in nature and is attributed to the reduction of metal in
the ‘tet-a’ compartment. The second reduction potential is irrevers-
ible in nature and is attributed to the reduction of (ANO2) group in
the side arm [34]. The quasi-reversible nature of the first peak is
evident from the following criteria: (i) the anodic peak current is
very much smaller than the cathodic peak current (Ipc ꢅ Ipa) and
1.70–1.74 BM, which is close to the spin-only value of the cop-
per(II) ion [30]. The magnetic moment values of all the copper(II)
complexes are given in Table 5. The nickel(II) complexes are dia-
magnetic due to square-planar geometry around the metal [31].
3.4. Electrochemical properties of the complexes
The electrochemical properties of the complexes were studied
by cyclic voltammetry in dimethylformamide containing 10ꢀ1
M
tetra(n-butyl)ammonium perchlorate. The electrochemical data
are summarized in Tables 6 and 7.
(ii)
DE increases with increasing scan rate, and is greater than
60 mV.
3.4.1. Reduction process at negative potential
The cyclic voltammogram of copper(II) and nickel(II) complexes
were recorded in the potential range 0 to ꢀ1.8 V and shown in
MII ꢀ MI

G. Nirmala et al. / Journal of Molecular Structure 989 (2011) 91–100
97
the result it is observed that the reduction potential of the
N-substituted copper complexes shift anodically, i.e. the reduction
potential for the di-substituted copper(II) complex of the ligand L1
is ꢀ0.80 to ꢀ0.94 V and for the unsubstituted copper complex it is
ꢀ1.00 V [2,8]. The anodic shift of the complexes may be due (i) the
steric effect of the phenyl groups present in the macrocycle, which
causes a small distortion of the geometry of the copper(II) com-
plexes (due to macrocyclic distortion), (ii) the tertiary nitrogen in
L1ꢀ3, causes more distortion of the coordination geometry around
the metal ion and makes the system more flexible, which stabilize
the low valent Cu(I) [6,35,36].
Thus the N-alkylation of the macrocyclic frame work was
shown to stabilize the monovalent metal ions by increasing the
cavity size of the macrocyclic ligand and by decreasing the ligand
field strength imparted by the nitrogen donor atom [35,37]. Similar
trend was also observed for the nickel(II) complexes with slight
variation in the reduction potential.
Fig. 4. Cyclic voltammogram of the [CuL1](ClO4)2 complex (reduction process).
Fig. 5. Cyclic voltammogram of the [CuL2](ClO4)2 complex (reduction process).
Fig. 6. Cyclic voltammogram of the [NiL1](ClO4)2 complex (oxidation process).
3.4.2. Oxidation process at positive potential
The cyclic voltammogram for nickel(II) complexes were re-
corded at anodic potential in the range 0 to +1.6 V. Each voltammo-
gram shows one quasi-reversible oxidation wave [29,32,33] at a
positive potential in the range +0.92 to +1.21 V. The controlled po-
tential electrolysis carried out at 100 mV more negative than the
anodic (oxidation) wave conveys the consumption of one electron
per molecule. The oxidation potential of nickel(II) complexes
shifts towards more positive potential value [29,32,33] upon
N-alkylation. Since N-substitution stabilizes the lower oxidation
state, the oxidation of [NiL1ꢀ3](ClO4)2 complex is more difficult
than that of the unsubstituted nickel(II) complex.
From the present study and the literature reports established
that a decrease in the ligand field strength of the macrocyclic li-
gand results in an anodic shift of the reduction potentials of their
metal complexes [29,32,33]. Alternatively the anodic shift in the
reduction potential of the nickel(II) complexes could be attributed
to the tetrahedral distortion of the coordination sphere caused by
the steric strain induced by the N-alkylation [28].
NiII ꢀ NiIII
3.5. Kinetic studies
3.5.1. Oxidation of pyrocatechol (catecholase activity)
The catecholase activity of the copper(II) complexes was carried
out using pyrocatechol as the convenient model substrate for the
identification of functional models for the metalloenzymes
[38,39]. For this purpose, 10ꢀ3 mol dmꢀ3 solutions of complexes
in dimethylformamide were treated with 100 equivalents of pyro-
catechol in the presence of air. The course of the reaction was
The comparison of electrochemical behavior of trans
N-substituted tet-a based copper(II) complexes with that of
unsubstituted tet-a based copper(II) complexes is interesting. From
Fig. 7. Catecholase activity of the copper(II) complexes (a) [CuL3](ClO4)2 (b)
[CuL1](ClO4)2.

98
G. Nirmala et al. / Journal of Molecular Structure 989 (2011) 91–100
Table 8
On comparison of the hydrolysis of 4-nitrophenylphosphate by
copper(II) and nickel(II) complexes, the nickel(II) complexes show
higher rate-constant values than the copper(II) complexes. It is
noteworthy that the reactivity of the complexes differs signifi-
cantly by introducing electron-withdrawing group on side arm.
The structural features and electrochemical properties are impor-
tant factors in determining the catalytic activity of the complexes.
The complexes of the ligands L2&3 which contains additional ANO2
group in the side arm are reduced at lower reduction potential and
shows higher catalytic activity. The complexes of ligands L2&3
shows red shift in the wavelength of absorption compared to com-
plexes of ligand L1. This red shift in the wavelength indicates that
the coordination geometry in the N-substituted complex is more
distorted than that of the unsubstituted complexes. It is evident
from the literature [40,41], that the rate constant for more dis-
torted complexes is higher than that of the less-distorted com-
plexes. At the same time, it should also be noted that the
catalytic activity of the complex containing electron-withdrawing
group is higher than that of the complex containing no electron-
withdrawing group. It can be seen that if the reduction potential
is too negative, the complex has a decreased catalytic activity
due to a more difficult reduction to metal(I). Literature reports,
[42] also show that the complexes containing electron-
withdrawing groups show higher catalytic activity than complexes
with electron donating substituents.
Hydrolysis of 4-nitrophenyl phosphate and catecholase activity of copper(II) and
nickel(II) complexes.
No
Complexes
Rate constant (k) (ꢂ10ꢀ3) minꢀ1
Catecholase
NPP
1
2
3
4
5
6
[CuL1](ClO4)2
[CuL2](ClO4)2
[CuL3](ClO4)2
[NiL1](ClO4)2
[NiL2](ClO4)2
[NiL3](ClO4)2
7.10
7.24
8.16
–
4.87
6.81
9.55
7.93
–
8.97
–
11.81
followed spectrophotometrically at 390 nm for nearly 45 min at
regular time intervals of 5 min. The slope was determined by the
method of initial rates for the formation of the product o-quinone.
A linear relationship for initial rate and the complex concentration
obtained for all the copper(II) complexes shows a first-order
dependence on the complex concentration for the systems. Plots
of log(A /A1 ꢀ At) versus time for catecholase activity of the cop-
1
per(II) complexes are obtained (Fig. 7) and the observed initial
rate-constant values is given in Table 8. The observed initial rate
constant value for the complexes is in the range of 7.12–
8.16 ꢂ 10ꢀ3 minꢀ1
.
3.5.2. Hydrolysis of 4-nitrophenylphosphate
3.6. Antimicrobial activities
The catalytic activity of the copper(II) and nickel(II) complexes
on the hydrolysis of 4-nitrophenylphosphate was determined
spectrophotometrically by monitoring the increase in the charac-
teristic absorbance of the 4-nitrophenolate anion at 420 nm at
25 °C. For this purpose, 10ꢀ3 mol dmꢀ3 solutions of complexes in
dimethylformamide were treated with 100 equivalents of 4-nitro-
phenylphosphate in the presence of air and the course of the reac-
tion was followed for nearly 45 min at regular time intervals. The
slope was determined by the method of initial rates by monitoring
the growth of 4-nitrophenolate anion. A linear relationship for all
the complexes shows a first-order dependence on the complex
concentration for the systems. Plots of log(A1/A1 ꢀ At) versus time
for hydrolysis of 4-nitrophenylphosphate activity of the complexes
were obtained and shown in Fig. 8. The observed initial rate-
constant values for all the copper(II) and nickel(II) complexes are
given in Table 8. The catalytic activity is in the range of
Antifungal and antibacterial activities of the complexes were
tested by the well diffusion method using Sabouraud dextrose agar
and Muller Hinton agar [43]. The radial growth of the colony was
recorded on completion of the incubation, and the mean diameter
for each complex at a concentration of 100 lg/ml was recorded.
The average percentage inhibition of the bacterial growth medium
was compared using the Vincent equation: I = 100 (C–T)/C, where
I = percentage inhibition, T = average diameter of the bacterial
growth on the tested plates and C = average diameter of the growth
on the control plates. Stock solutions of tested compounds were
prepared in dimethyl sulfoxide. The diameters of the zone of inhi-
bition produced by the compounds were compared with the stan-
dard antibiotics Streptomycin 10 lg/well and Azithromycin 15 lg/
well (for Candida albicans).
4.87 ꢂ 10ꢀ3 to 11.81 ꢂ 10ꢀ3 minꢀ1
.
3.6.1. Inoculum preparation
Fresh bacterial cultures were used for the antibacterial suscep-
tibility test. Five ATCC colonies of the strains were inoculated to
Tryptic soy or Brain Heart Infusion broth and incubated at 37 °C
for 22–24 h time period. Turbidity was adjusted with sterile broth
so as to correspond to the 0.5 Mc Farland standard, a standard
inoculum of the microorganism 1.5 ꢂ 106 colony forming units
CFU/ml, a 1:100 dilution of a suspension of turbidity equal to a
Mc Farland standard 0.5. The turbidity was adjusted to match a
Mc Farland 0.5 barium sulfate method. This is prepared by adding
0.5 ml of 1.175% w/v (0.048 m) hydrate (BaCl2.2H2O) to 99.5 ml of
1% w/v (0.36) sulfuric acid.
3.6.2. Antifungal activity
We have evaluated the antifungal activity of all the ligands and
their copper(II) and nickel(II) complexes against the human patho-
genic fungus C. albicans. The screening data for the complexes are
given in Table 9. It is observed from the results that all copper(II)
and nickel(II) complexes show some antifungal activity. However,
the complexes [NiL2](ClO4)2 and [NiL3](ClO4)2 showed remarkable
activity against the tested fungus at concentration of 100 lg/ml
when compared with the standard azithromycin. The activity
observed by the present complexes was comparable with the
Fig. 8. Hydrolysis of 4-nitrophenylphosphate by (a) [NiL1](ClO4)2 (b) [NiL2](ClO4)2.

G. Nirmala et al. / Journal of Molecular Structure 989 (2011) 91–100
99
Table 9
bial activity has been investigated. Whatever the precise interpre-
tation in the electrochemical, catalytic and antimicrobial activity of
the N-functionalized macrocyclic ligands, their magnitude is sub-
ject to vary due to the change caused by the nature of the ligand,
pendant groups and metal salt used for complexation.
Antibacterial activity of N-substituted ‘‘tet-a’’ based copper(II) and nickel(II)
complexes.
Sample
Representation zone of inhibition (diameter in mm)
Antibacterial activity
Antifungal activity (100 lg/ml)
(100 lg/ml)
S.a
B.s
K.p
P.a
E.c
C.a
Acknowledgements
[CuL1](ClO4)2 14
[CuL2](ClO4)2 21
[CuL3](ClO4)2 21
[NiL1](ClO4)2 16
[NiL2](ClO4)2 19
[NiL3](ClO4)2 18
15
25
21
20
21
24
19
18
15
15
19
20
19
19
24
19
20
19
20
12
16
16
18
26
16
18
18
17
20
21
The authors are grateful for the financial supports from the
University Grants Commission and Council of Scientific and
Industrial Research, New Delhi, India.
Appendix A. Supplementary material
S.a = Staphylococcus aureus (ATCC 12600).
B.s = Bacillus subtilis (ATCC 6633).
K.p = Klebsiella pneumonia (ATCC 13883).
CCDC 672049 contains the supplementary crystallographic data
for ligand L1. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, (or from the
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax:+ 44 1223 336 033).
P.a = Pseudomonas aeruginosa (ATCC 10145).
E.c = Escherichia coli (ATCC 11775).
C.a = Candida albicans.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.molstruc.2011.01.010.
N-substituted tetraazamacrocycles reported in the literature [44].
It is also observed from this study that the mononuclear complexes
containing aromatic nitro groups show higher activity than the
other complexes.
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3.6.3. Antibacterial activity
All the prepared ligands and their complexes have been
screened for their invitro antibacterial activity against selected five
pathogenic bacteria such as Pseudomonas aeruginosa (ATCC 10145),
Staphylococcus aureus (ATCC 12600), Escherichia coli (ATCC 11775),
Bacillus subtilis (ATCC 6633), and Klebsiella pneumonia (ATCC
13883), respectively. The screening results of the complexes are
shown in the Tables 9. All the copper(II) and nickel(II) complexes
show comparable activity against all the selected bacteria. From
the data it is observed that at concentration of 100 lg/ml, the com-
plexes [CuL1](ClO4)2 and [NiL3](ClO4)2 showed higher antibacterial
activity against E. coli in comparison with the standard antibiotic
streptomycin. Similarly the complexes [CuL3](ClO4)2 and [NiL2](-
ClO4)2 were active against P. aeruginosa, [NiL3](ClO4)2 was active
against K. pneumonia, whereas the complexes [CuL2](ClO4)2 and
[CuL3](ClO4)2 were active against S. aureus in comparison with
the standard antibiotic streptomycin. All the complexes except
[CuL1](ClO4)2 were active against B. subtilis. The higher activity of
the complexes may be ascribed to Tweedy’s theory, according to
which chelation reduces the polarity of the central metal atom be-
cause of partial sharing of its positive charge with the ligand, which
favors the permeation of the complexes through the lipid layer of
the membrane [45]. It seems from the results that the nature of
the ligand and the coordinated metal ion plays a significant role
in the inhibition activity.
The antibacterial activity is dependant on the molecular
structure of the compound, the solvent used, and the bacterial
strain under consideration. The complexes containing electron-
withdrawing group shows higher activity than the complexes of
ligand L1. The comparatively lower activity of certain complexes
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electron-withdrawing group may be because of the lower lipophil-
icity. This may decrease the penetration of the complexes through
the lipid membrane, which could neither block nor inhibit the
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