Synthesis and characterization of some new Cd(II) and Zn(II) macrocyclic Schiff base complexes derived from cyclocondensation of three new linear aromatic (N4) amines and 2,6-diacetylpyridine. Crystal structure and biological activity studies

Article abstract of DOI:10.1007/s13738-014-0417-7

Six new Cd(II) and Zn(II) macrocyclic Schiff base complexes from cyclocondensation of 2,6-diacetylpyridine and three different liner aromatic amines have been investigated. The resulting ligands, L¹ and L ² are 17- and L³ 18-membered pentaaza macrocycles. The complexes have been characterized by a variety of methods such as IR, elemental analysis, EI-Mass and conductivity measurements and in the case of cadmium complex by ¹H and ¹³C NMR spectroscopy. The crystal structure of [CdL¹(CH₃OH)](ClO₄)₂ has been determined. In this complex, the Cd(II) ion is coordinated to five nitrogen donor atoms of the ligand and one oxygen atom from methanol in a N ₅O environment. The geometry around the cadmium can be better described as in a slightly distorted pentagonal-pyramidal. The synthesized compounds have antibacterial activity against the three gram-positive bacteria: S. aureus, B. cereus and C. xerosis, and also against the three gram-negative bacteria: E. coli, K. pneumoniae and P. vulgaris. The results show that the antibacterial activity of cadmium complexes is greater than that of zinc complexes.

Full text of DOI:10.1007/s13738-014-0417-7

J IRAN CHEM SOC
DOI 10.1007/s13738-014-0417-7
ORIGINAL PAPER
Synthesis and characterization of some new Cd(II)
and Zn(II) macrocyclic Schiff base complexes derived from
cyclocondensation of three new linear aromatic (N4) amines
and 2,6-diacetylpyridine. Crystal structure
and biological activity studies
•
Hassan Keypour Mozhdeh Liyaghati-Delshad
•
•
Majid Rezaeivala Hamid Reza Khavasi
Received: 11 November 2013 / Accepted: 15 January 2014
Ó Iranian Chemical Society 2014
Abstract Six new Cd(II) and Zn(II) macrocyclic Schiff
base complexes from cyclocondensation of 2,6-diacetyl-
pyridine and three different liner aromatic amines have
been investigated. The resulting ligands, L1 and L2 are 17-
and L3 18-membered pentaaza macrocycles. The com-
plexes have been characterized by a variety of methods
such as IR, elemental analysis, EI-Mass and conductivity
activity of cadmium complexes is greater than that of zinc
complexes.
Keywords Macrocyclic ligand ꢀ Schiff base ꢀ X-ray
crystal structure ꢀ Tetradentate amine ꢀ Biological activity
1
measurements and in the case of cadmium complex by H
Introduction
and 13C NMR spectroscopy. The crystal structure of
[CdL1(CH3OH)](ClO4)2 has been determined. In this
complex, the Cd(II) ion is coordinated to five nitrogen
donor atoms of the ligand and one oxygen atom from
methanol in a N5O environment. The geometry around the
cadmium can be better described as in a slightly distorted
pentagonal-pyramidal. The synthesized compounds have
antibacterial activity against the three gram-positive bac-
teria: S. aureus, B. cereus and C. xerosis, and also against
the three gram-negative bacteria: E. coli, K. pneumoniae
and P. vulgaris. The results show that the antibacterial
Over the past few decades, macrocyclic compounds have
become important synthetic targets due to their wide
applications in molecular recognition, supramolecular
structures, catalysis, and medicine [1–9]. One main repre-
sentative group of macrocycles is the Schiff base macro-
cyclic compound family. The Schiff base macrocycles are
synthesized by the reaction of dicarbonyl compounds with
diamines by well-understood mechanisms [10, 11]. Metal
template synthesis of multidentate and macrocyclic ligands
offers high-yielding and selective routes to new ligands and
their complexes [12, 13]. A template effect may arise from
the stereochemistry imposed by metal ion coordination of
some of the reactants, promoting a series of controlled
steps which provides routes to products that do not offer
form in the absence of metal ion [14]. The relationship
between the structure and the chelate ring size of multid-
entate ligands in coordination compounds is a subject of
considerable importance because the coordination chem-
istry of the metals is affected by the both the type of donor
atom and steric requirements [15]. Pyridine and its deriv-
atives find application in areas where bioactivity is
important, as in medicinal drugs and in agricultural pro-
ducts such as herbicides, insecticides, fungicides, and plant
growth regulators [16]. We and others have been interested
for some time in the design and synthesis of macrocyclic
Schiff base complexes from metal ion templated [1 ? 1]
Electronic supplementary material The online version of this
article (doi:10.1007/s13738-014-0417-7) contains supplementary
material, which is available to authorized users.
H. Keypour (&) ꢀ M. Liyaghati-Delshad
Faculty of Chemistry, Bu-Ali Sina University,
65174 Hamedan, Iran
e-mail: haskey1@yahoo.com
M. Rezaeivala (&)
Department of Chemical Engineering, Hamedan University
of Technology, 65155 Hamedan, Iran
e-mail: mrezaeivala@hut.ac.ir; mrezaeivala@gmail.com
H. R. Khavasi
Department of Chemistry, Shahid Beheshti University,
G.C., Evin, 1983963113 Tehran, Iran
123

J IRAN CHEM SOC
O
cyclocondensation of 2,6-diacetylpyridine with these amines
to give the pentadentate ligands L1, L2 and L3 (Scheme 2).
The resulting complexes were characterized by IR spectra,
elemental analysis and molar conductance and in the case of
NO2
NH2
CH
R
(CH2)n
NH2
1
cadmium complex, by H and 13C NMR spectroscopy. The
2
+
crystal structures of [CdL1(CH3OH)](ClO4)2 is also reported.
The synthesized compounds have antibacterial activity
against the three Gram-positive bacteria: S. aureus, B. cereus
and C. xerosis, and also against the three Gram-negative
bacteria: E. coli, K. pneumoniae and P. vulgaris.
O2N
NO2
Experimental
Chemical and starting materials
NH
CH
R
(CH2)n
NH
All solvents were of reagent grade quality and purchased
commercially.
Ethane-1,2-diamine,
propane-1,2-
diamine,propane-1,3-diamine, 2-nitrobenzaldehyde, 2,6-
diacetylpyridine, sodium borohydride and metal salts were
purchased from Merck and used without further purification.
Caution: Perchlorate salts are potentially explosive.
While we have not experienced any problems with the
compounds described, they should be treated with caution
and handled in small quantities.
H2N
NH2
General synthesis of the tetradentate amines (1–3)
The general method for the synthesis of the tetradentate
amines is as follows: A solution of ethane-1,2-diamine
(0.60 g, 10 mmol), propane-1,2-diamine (0.74 g, 10 mmol)
and propane-1,3-diamine (0.75 g, 10 mmol) in dry MeOH
(30 ml) was added dropwise to a warm solution of
2-nitrobenzaldehyde (3.02 g, 20 mmol) in dry methanol
(30 ml) over a period of 2 h. The mixture was refluxed
under stirring for 24 h and then allowed to cool to room
temperature. Solid sodium borohydride (3.02 g, 80 mmol)
was added slowly and the reaction mixture heated at reflux
for a further 2 h. The solution was filtered and the filtrate
volume reduced to 20 ml by rotary evaporation. Excess
water was added and the product was extracted with chlo-
roform (3 9 25 ml). The combined chloroform solutions
were separated and dried over magnesium sulfate. The
chloroform was removed by rotary evaporation to leave
yellow oil (90–95 %). The resulting product (10 mmol) was
dissolved in EtOH:H2O (200:20 ml), and NH4Cl (13.91 g,
26 mmol) was added. The mixture was refluxed and then
zinc dust (3.217 g, 12 mmol) was added slowly. The mix-
ture was refluxed for 12 h. When the solution was pale
white it was filtered. The volume was then reduced to 20 ml
by rotary evaporation. Then water (50 ml) was added to the
mixture and the pH was adjusted to 12 with sodium
hydroxide. The product was extracted with chloroform
NH
CH
R
(CH2)n
NH
n= 1 , R= H (1)
n=1 , R=CH3 (2)
n=2 , R=H
(3)
Scheme 1 Synthesis of the tetradentate amines (1–3)
cyclocondensation of 2,6-diacetylpyridine and various tri-
podal, linear or branched polyamines [17–24]. Also, we are
recently reported the biological activities of synthesized
complexes [24–28]. As an extension of this area, in this
paper first we report the synthesis and characterization of
three new linear aromatic amines, N,N0-bis(2-aminoben-
zyl)ethane-1,2-diamine (1), N,N0-bis(2-aminobenzyl)pro-
pane-1,3-diamine (2) and N,N0-bis(2-aminobenzyl)-
propane-1,3-diamine (3) (Scheme 1). These amines were
prepared from the reaction of 2-nitrobenzaldehyde and
ethane-1,2-diamine, propane-1,2-diamine and propane-1,3-
diamine in methanol and reduction of both imines and nitro
groups by sodium borohydride and zinc dust, respectively
and also the Cd(II) and Zn(II) template [1 ? 1]
123

J IRAN CHEM SOC
Scheme 2 The template
condensation between 2,6-
diacetylpyridine and the
tetradentate amines in the
presence of Zn(II), Cd(II)
metal ions
NH2
H2N
(CH2)n
HC
R
N
+
N
H
N
O
O
H
(ii) M2+
(i) MeOH
N
N
N
M2+
(CH2)n
HC
N
N
H
H
R
L1, n= 1 , R= H
L2, n=1 , R=CH3
L3, n=2 , R=H
M= Zn, Cd
(3 9 25 ml), and then the combined chloroform solutions
were separated, and dried over magnesium sulfate. The
chloroform was removed by rotary evaporation to leave the
product as yellow solid (Scheme 1).
(CDCl3, ppm) d = 18.11 (C-3); 47.53 (C-1); 49.98 (C-5);
51.95 (C-4); 54.60 (C-2); 123.87 (C-10); 127.24 (C-8); 129.10
(C-6); 132.42 (C-9); 135.35 (C-7); 148.64 (C-11) (Scheme 3).
N,N0-Bis(2-aminobenzyl)propane-1,3-diamine (3)
N,N0-Bis(2-aminobenzyl)ethane-1,2-diamine (1)
Yield: 1.96 g (69 %). Anal. Calc. for C17H24N4 (MW:
284.20): C, 71.79; H, 8.51; N, 19.70. Found: 71.81; H, 8.52;
N, 19.67. IR (KBr, cm-1) 3315, 3251m(NH2), 3156 m(NH).
1H NMR(CDCl3, ppm) d = 2.08(CH2,2H); 2.46(NH,2H);
3.40(CH2,4H); 4.44(CH2,4H); 3.55(NH2, 4H); 7.36-7.93
(Ar, 8H). 13C NMR(CDCl3, ppm) d = 29.29 (C-1); 46.94
(C-2); 52.20 (C-3); 115.16 (C-8); 117.12 (C-6); 123.43 (C-
4); 127.86 (C-7); 129.42 (C-5); 146.36 (C-9).
Yield: 1.97 g (73 %). Anal. Calc. for C16H22N4 (MW:
270.37): C, 71.08; H, 8.20; N, 20.72. Found: 71.10; H,
8.21; N, 20.69. IR (KBr, cm-1) 3394, 3308m(NH2), 3203
1
m(NH). H NMR(CDCl3, ppm) d = 2.72 (CH2, 4H); 3.28
(NH2, 4H); 3.75 (CH2, 4H); 4.31(NH, 2H); 6.57–7.21 (Ar,
8H). 13C NMR(CDCl3, ppm) d = 48.61 (C-2); 52.52 (C-
1); 115.48 (C-7); 117.42 (C-5); 123.90 (C-3); 128.09 (C-6);
129.57 (C-4); 146.55 (C-8) (Scheme 3).
Synthesis of the metal complexes; general procedure
N,N0-Bis(2-aminobenzyl)propane-1,2-diamine (2)
The complexes were readily prepared by reaction of
methanolic solution of amines (1–3) (0.5 mmol) and 2,6-
diacetylpyridine (0.5 mmol) in the presence of appropriate
metal salts (0.5 mmol). The reaction was carried out for
24 h at reflux (Scheme 2). The solution volume was
reduced to 10 ml by rotary evaporation and a small volume
Yield: 2.14 g (75 %). Anal. Calc. for C17H24N4 (MW: 284.40):
C, 71.80; H, 8.50; N, 19.70. Found: 71.83; H, 8.51; N, 19.66. IR
(KBr, cm-1
)
3393, 3309m(NH2), 3206 m(NH). 1H
NMR(CDCl3, ppm) d = 1.80(CH3, 3H); 2.60(CH, CH2,
NH,8H); 3.74(NH2, 4H); 6.67–7.13 (Ar, 8H). 13C NMR
123

J IRAN CHEM SOC
7
13
H2N
NH2
8
12
11
6
5
10
7
9
N
3
HN
NH
4
2
N
N
8
6
5
Cd2+
1
(1)
3
10
7
4
HN
NH
H2N
NH2
2
11
6
9
1
8
15
HN
NH
1
5
14
13
3
4
12
2
11
N
(2)
9
N
N
10
8
Cd2+
8
5
NH2
H2N
9
7
5
7
1
H
N
H
HN
NH
6
N
6
4
4
3
2
3
2
1
(3)
Scheme 3 Schematic structures of amines (1–3) showing the
numbering scheme for NMR assignments
14
13
12
of diethyl ether was slowly added to the solution,
producing a powdery precipitate. The product was filtered
off, washed with cold diethyl ether and dried under
vacuum.
11
N
10
8
N
N
Cd2+
9
4
7
[ZnL1](ClO4)2
H
H
N
N
1
6
Yield: 0.18 g (54 %). Anal. Calc. for C25H27ZnCl2N5O8 (MW:
5
2
661.80) C, 45.37; H, 4.11; N, 10.58. Found: C, 45.28; H, 4.21;
3
N,10.46. %.IR (KBr, cm-1) 3262 m(N–H)], 1632 m(C=N)imi
.
1593, 1463 [m(C=C) and m(C=N)py] and 1097, 623 m(ClO4-).
EI-Mass: (m/z, M?) 562 [ZnL4]ClO4?. Km/X-1 cm2 mol-1
(in DMSO): 175 (2:1).
Scheme 4 Schematic structures of cadmium(II) cation complexes
showing the numbering scheme for NMR assignments
623 m(ClO4-). EI-Mass: (m/z, M?) 576 [ZnL5]ClO4
Km/X-1 cm2 mol-1 (in DMSO): 171 (2:1).
.
?
[ZnL2](ClO4)2
[ZnL3](ClO4)2
Yield: 0.20 g (60 %). Anal. Calc. for C26H29ZnCl2N5O8
(MW: 675.82) C, 46.21; H, 4.33; N, 10.36. Found: C, 46.08;
H, 4.44; N,10.22. %. IR (KBr, cm-1) 3280 m(N–H)], 1630
m(C=N)imi. 1584, 1454 [m(C=C) and m(C=N)py] and 1096,
Yield: 0.18 g (53 %). Anal. Calc. for C26H29ZnCl2N5O8
(MW: 675.82) C, 46.21; H, 4.33; N, 10.36. Found: C, 46.15;
123

J IRAN CHEM SOC
H, 4.38; N, 10.29. %.IR (KBr, cm-1) 3271 m(N–H)], 1639
m(C=N)imi. 1592, 1466 [m(C=C) and m(C=N)py] and 1097,
623 m(ClO4-). EI-Mass: (m/z, M?) 576 [ZnL6]ClO4?. Km/
X-1 cm2 mol-1(in DMSO): 169 (2:1).
Table 1 Crystal data and structure refinement for [CdL1(CH3OH)]
(ClO4)2
[CdL1(CH3OH)](ClO4)2
Empirical formula
Formula weight
Temperature (K)
C26H31CdCl2N5O9
740.87
[CdL1](ClO4)2
298(2)
˚
Wavelength (A)
0.71073
Yield: 0.22 g (62 %). Anal. Calc. for C25H27CdCl2N5O8
(MW: 708.83) C, 42.36; H, 3.84; N, 9.88. Found: C, 42.18;
H, 4.19; N, 9.47. %. IR (KBr, cm-1) 3277 m(N–H)], 1634
m(C=N)imi. 1588, 1463 [m(C=C) and m(C=N)py] and 1100,
623 m(ClO4-). EI-Mass: (m/z, M?) 610 [CdL4]ClO4?. Km/
X-1 cm2 mol-1 (in DMSO): 168 (2:1).
Crystal system, space group
Monoclinic, Cc
8.2758 (8)
22.0818 (18)
16.5795 (15)
90
˚
a (A)
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
F(000)
93.062 (8)
90
[CdL2](ClO4)2
1504
Crystal size (mm3)
Absorption coefficient (mm-1
Limiting indices
0.40 9 0.25 9 0.23
0.958
Yield: 0.19 g (52 %). Anal. Calc. for C26H29CdCl2N5O8
(MW: 722.85) C, 43.20; H, 4.04; N, 9.69. Found: C, 43.15;
H, 4.13; N, 9.59. %. IR (KBr, cm-1) 3269 m(N–H)], 1629
m(C=N)imi. 1587, 1465 [m(C=C) and m(C=N)py] and 1096,
623 m(ClO4-). EI-Mass: (m/z, M?) 624 [CdL5]ClO4?. Km/
X-1 cm2 mol-1 (in DMSO): 165 (2:1).
)
-11 B h B 9, -30 B k B 26,
-22 B l B 22
Theta range for data collection (°) 2.22–29.20
Max. and min. transmission
0.8541 and 0.7439
Full-matrix least-squares on F2
Refinement method
[CdL3](ClO4)2
Data/restraints/parameters
Goodness-of-fit on F2
6973/3/401
0.898
Final R indices [I [ 2r (I)]
R indices (all data)
Largest diff. peak and hole (e.A-3
R1 = 0.0395, wR2 = 0.0878
R1 = 0.0679, wR2 = 0.1085
0.315 and -0.267
Yield: 0.17 g (47 %). Anal. Calc. for C26H29CdCl2N5O8
(MW: 722.85) C, 43.20; H, 4.04; N, 9.69. Found: C, 43.11;
H, 4.17; N, 9.54. %. IR (KBr, cm-1) 3225 m(N–H)], 1642
m(C=N)imi. 1591, 1465 [m(C=C) and m(C=N)py] and 1091,
624 m(ClO4-). EI-Mass: (m/z, M?) 623 [CdL6]ClO4?. Km/
X-1 cm2 mol-1 (in DMSO): 162 (2:1).
)
[CdL1(CH3OH)](ClO4)2 was performed on STOE IPDS-II
two-circle diffractometer, using graphite monochromated
Mo Ka X-ray radiation (k = 0.7107 nm). The data col-
lection was performed at room temperature using the
x-scan technique and using the STOE X-AREA software
package [29]. The crystal structures were solved by direct
methods [30] and refined using X-STEP32 crystallographic
software package [31]. All of the non-hydrogen atoms were
refined anisotropically. All of hydrogen atoms were located
in ideal positions.
Physical measurements
Elemental analyses were performed in a Carlo-Erba EA
microanalyser. IR spectra were measured on a Perkin
Elmer FT-IRGX spectrophotometer in the range of
400–4000 cm-1 by KBr pellet technique. Conductivimetry
measurements were carried out in 10-3 mol dm-3 dim-
ethylsolfoxide solutions at 25 °C using a CARISON
GLP32 conductivimeter. EI-mass spectra were recorded
1
using 5973 Network Mass Selective Detector. H and 13C
Antibacterial study
NMR spectra were recorded on a Bruker 400 MHz using
DMSO as solvent.
Materials and methods
X-ray crystal structure determination
Six bacterial strains, three gram positive and three gram
negative, were selected on the basis of their clinical
importance in causing diseases in humans Table 2. Some
microorganisms were obtained from Persian Type Culture
Collection, Tehran, Iran and others locally isolated (Lio).
The strains selected for the study are S. aureus (ATCC
6633), B. cereus (ATCC 7064), C. xerosis (ATCC 373)
(Gram-positive bacterial strains), E. coli (PTCC 10009), K.
Vapor diffusion of diethyl ether into a solution of
[CdL1(CH3OH)](ClO4)2 in a mixture of acetonitrile and
methanol afforded a pale yellow crystalline needles suit-
able for study by X-ray crystallography. The details of the
X-ray crystal data, and of the structure resolution and
refinement, are given in Table 1. X-ray data for
123

J IRAN CHEM SOC
Table 2 Minimum inhibitory concentration (MIC in mg/ml) of the macrocyclic complexes
S.no
Bacterial strains
Complexes
[CdL1](ClO4)2
[ZnL1](ClO4)2
[CdL2](ClO4)2
[ZnL2](ClO4)2
[CdL3](ClO4)2
[ZnL3](ClO4)2
1.
2.
3.
4.
5.
6.
S. aureus
B. cereus
C. xerosis
E. coli
0.653
0.881
0.401
1.012
0.940
1.432
1.546
1.784
1.432
1.222
2.012
2.023
0.587
0.819
0.367
0.845
0.832
1.013
1.325
1.514
1.245
0.985
1.847
1.954
0.574
0.786
0.321
0.814
0.792
0.974
1.258
1.342
1.214
0.964
1.856
1.952
K. pneumoniae
P. vulgaris
pneumoniae (MTCC 109), and P.vulgaris (lio) (Gram-
negative bacterial strains). These strains were screened for
evaluation of antibacterial activities of the synthesized
complexes.
the micro-titer plates. In sterile microtitre plates (96-u-
shaped wells) 50 ll of the sterile nutrient broth was poured
in each well in three rows, then from fresh inoculums so
formed (108 c.f.u./ml diluted with 100 ll nutrient broth to
have 106c.f.u./ml) 50 ll of the suspension was poured in
each well in the first and third row, second row was again
filled with 50 ll of nutrient broth, finally the drug sample
50 ll was added in the first row diluting uniformly from
2.5 to 0.01 mg/ml till the 8th well. All the microtitre
plates were incubated at 37 °C for 18–24 h. MIC was
expressed as the lowest dilution, which inhibited the
growth of bacteria observed by lack of turbidity in the
well.
Primary screening
The antibacterial activity of the macrocyclic complexes
was evaluated by agar well diffusion method [32]. All the
microbial cultures were adjusted to 0.5 McFarland stan-
dards, which is visually comparable to a microbial sus-
pension of approximately 1.5 9 108 c.f.u./ml [33]. 20 ml
of agar media was poured into each petri plate these plates
were then swabbed with a colony from inoculum of the test
microorganisms and kept to adsorption for 15 min. Using
sterile cork borer of 6 mm diameter, wells were bored into
the seeded agar plates and these were loaded with 50 ll
volume with concentration of 10 mg/ml of each compound
reconstituted in DMSO. All the plates were incubated at
37 °C for 24 h. Antibacterial activity of all the complexes
was evaluated by measuring the diameter of zone of inhi-
bition in mm (Table 3). The medium with DMSO as sol-
vent was used as a negative control, whereas media with
ciprofloxacin (standard antibiotic for gram positive) and
gentamicin (standard antibiotic for gram negative) were
used as positive control. The experiments were performed
in triplicates.
Results and discussion
Synthesis and characterization
The Schiff base macrocyclic complexes were prepared
from [1 ? 1] cyclocondensation of 2,6-diacetylpyridine
and N,N0-bis(2-aminobenzyl)ethane-1,2-diamine (1) or
N,N0-bis(2-aminobenzyl)propane-1,3-diamine (2) or N,N0-
bis(2-aminobenzyl)propane-1,3-diamine(3) in the presence
of Cd(II) and Zn(II) metal ions. The complexes were
characterized by IR, elemental analysis, mass and in the
1
case of cadmium complexes by H and 13C NMR spec-
troscopy. The solid compounds are air stable and elemental
analyses are consistent with the formulations given in
‘‘Experimental’’. An important feature is the occurrence of
a strong band at approximately 1,629–1,642 cm-1 attrib-
utable to C=N stretching modes, indicating the Schiff base
condensation and also bands at ca. 1,590 and 1,460 cm-1
associated with t(C=N)py and t(C=C) vibrations from the
pyridine and phenyl rings [34] are present in the spectra,
but no bands for the free carbonyls or primary diamines are
observed indicating that complete condensation has
occurred. The absorptions attributable to ionic perchlorate
were found at approximately 1,090 and 623 cm-1 [35].
The lack of splitting these bands suggests that the per-
chlorate anions are not coordinated [36].
Determination of minimum inhibitory concentration (MIC)
Minimum inhibitory concentration (MIC) is the lowest
concentration of an antimicrobial compound that will
inhibit the visible growth of microorganisms after over-
night incubation. Minimum inhibitory concentrations are
important in diagnostic laboratories to confirm resistance
of microorganisms to antimicrobial agents and also to
monitor the activity of new antimicrobial agents. The MIC
of the macrocyclic Schiff base complexes was tested
against bacterial strains through a broth-dilution method. In
this method, the test concentrations of all the complexes
were made from 2.5 to 0.01 mg/ml in the sterile wells of
123

J IRAN CHEM SOC
Table 3 Inhibition zones (mm) of complexes against bacterial strains
Complex
Bacteria
S. aureus
B. cereus
C. xerosis
E. coli
K. pneumoniae
P. vulgaris
[CdL1](ClO4)2
[ZnL1](ClO4)2
[CdL2](ClO4)2
[ZnL2](ClO4)2
[CdL3](ClO4)2
[ZnL3](ClO4)2
Ciprofloxacin
Gentamicin
25
15
26
17
29
22
30
–
15
10
17
14
21
20
27
–
20
15
22
17
26
22
27
–
15
10
17
12
22
18
–
18
10
19
11
23
16
–
20
12
22
15
25
19
–
25
25
25
X-ray crystal structure
By slow recrystallization of the compound in mixture of
methanol/acetonitrile, crystals of [CdL1(CH3OH)](ClO4)2
suitable for X-ray diffraction were obtained. The molecular
structure of the [CdL1(CH3OH)](ClO4)2 and selected bond
lengths
and
angles
are
shown
in
Fig. 1.
[CdL1(CH3OH)](ClO4)2 crystallize in the monoclinic Cc
space group. Crystal structure of the complex as shown
represents the Cd(II) ion with a N5O coordination envi-
ronment. A similar six-coordinate geometry has been found
for other Cd(II) complexes with N5 macrocyclic ligands
[37]. The geometry around the metal can be better
described as in a slightly distorted pentagonal-pyramidal.
The ligand shows a nearly plane conformation and the rms
of the plane formed by the five N atoms is 0.667, showing
that these atoms are almost coplanar, with Cd(II) ion 0.213
Fig. 1 View of the [CdL1(CH3OH)]2? cation: selected bond lengths
(A) and angles (°): N(1)-Cd(1) 2.292(5); N(2)-Cd(1) 2.353(4); N(3)-
˚
˚
A out of this plane, while the apex of the pyramid is
Cd(1) 2.253(4); N(4)-Cd(1) 2.366(4); N(5)-Cd(1) 2.285(5); O(1)-
Cd(1) 2.463(6); N(3)-Cd(1)-N(5) 132.69(19); N(3)-Cd(1)-N(1)
147.41(17); N(5)-Cd(1)-N(1) 77.73(19); N(3)-Cd(1)-N(2) 70.45(16);
N(5)-Cd(1)-N(2) 118.82(19); N(1)-Cd(1)-N(2) 85.01(17); N(3)-
Cd(1)-N(4) 70.27(16); N(5)-Cd(1)-N(4) 83.89(17); N(1)-Cd(1)-N(4)
134.00(17); N(2)-Cd(1)-N(4) 140.06(15); N(3)-Cd(1)-O(1) 84.3(2);
N(5)-Cd(1)-O(1) 130.7(2); N(1)-Cd(1)-O(1) 80.3(2); N(2)-Cd(1)-
O(1) 102.33(19); N(4)-Cd(1)-O(1) 80.70(18)
occupied by the oxygen atom of methanol. The sum of five
N–Cd–N chelate angles (387.35) in an approximately
pentagonal plane is almost 3608 for an ideal planar struc-
ture. The pyridine group provides the shortest bond to the
˚
metal, Cd(1)-N(3) 2.253(4) A, while the longest belongs to
the imine nitrogen atoms, Cd(1)-N(2) 2.353(4) and Cd(1)-
˚
N(4) 2.366(4) A. The perchlorate ion giving typical bond
distances and angles, and the structure shows that it is not
coordinated to the Cd(II) ion reflecting the low coordina-
tion ability of this anion.
two imine environments. The 1H NMR spectrum of
[CdL1(CH3OH)] (ClO4)2 shows that the three pyridine
protons collapse in an overlapped broad signal at d
8.65 ppm as has been previously reported for Nelson and
1
NMR studies
co-workers [38]. In the aliphatic region of the H NMR
spectrum, two signals are observed in 2.50 and 3.37 due to
the CH2 protons belonging to the aliphatic protons of the
amine. The methyl groups belonging to the 2,6-diacetyl-
1
The H and 13C NMR data for three cadmium complexes
dissolved in DMSO-d6 are shown in Table 4, which uses
the lettering scheme shown in Scheme 4. The NMR spectra
of the complexes confirm the formation of fully condensed
macrocyclic Schiff base complexes. The 13C NMR spec-
trum of [CdL1(CH3OH)](ClO4)2, shows the imine reso-
nance at 158.73 ppm, demonstrating the equivalence of the
1
pyridine precursor appear at 1.23 ppm in H NMR and
18.25 ppm in 13C NMR. The 13C NMR spectrum of
[CdL2](ClO4)2 shows the imine resonance at 158.86 ppm.
Also, the methyl groups belonging to the 2,6-diacetylpyr-
idine precursor appear at d 0.93 ppm in 1H NMR and
123

J IRAN CHEM SOC
Table 4 1H and 13C{1H} NMR spectral assignments for cadmium complexes recorded at 400 MHz
1
Complex
Assignment hydrogen
atom for complex
(d, ppm) H NMR
in DMSO-d6
Integration
Assignment carbon
atom for complex
(d, ppm) 13C NMR
in DMSO-d6
[CdL1](ClO4)2
H1
2.50 (s)
4H
4H
8H
2H
3H
2H
C1
48.51
48.94
H2
3.37 (s)
C2
H4–H7
H10
6.99–7.41 (m)
1.23 (s)
C3
119.14
132.27
128.23
129.15
122.01
146.79
158.73
18.25
C4
H12, H13
8.65 (s)
C5
NH
4.08 (b)
C6
C7
C8
C9
C10
C11
C12
C13
C1
148.61
126.72
143.66
18.28
[CdL2](ClO4)2
H1
1.24 (s)
3H
1H
2H
4H
8H
6H
3H
2H
H2
2.90 (b)
2.64 (b)
3.43 (s)
C2
51.95
H3
C3
52.27
H4
C4
48.57
H6–H9
H12
6.98–7.28 (m)
0.93 (s)
C5
119.23
132.31
128.33
129.24
122.15
146.82
158.86
16.12
C6
H14, H15
8.79 (s)
C7
NH
4.10 (b)
C8
C9
C10
C11
C12
C13
C14
C1
148.72
126.80
32.33
[CdL3](ClO4)2
H1
1.23 (s)
2H
4H
4H
8H
6H
3H
2H
H2
2.44 (s)
C2
44.72
H3
3.81 (b)
6.83–7.28 (m)
0.84 (s)
C3
48.30
H5–H8
H11
C4
119.12
131.97
126.70
129.12
121.96
146.95
157.53
18.23
C5
H13, H14
8.78 (s)
C6
NH
4.00 (s)
C7
C8
C9
C10
C11
C12
C13
C14
148.73
127.03
124.67
16.12 ppm in 13C NMR. For [CdL3](ClO4)2, the 13C NMR
spectrum shows the imine resonance at d 157.53 ppm
demonstrating the equivalence of the two imine environ-
ments and in this case, the methylene groups belonging to
the 2,6-diacetylpyridine precursor appear at d 0.84 and
18.23in 1H and 13C NMR spectra, respectively. Zinc
complexes could not be studied by NMR due to their low
solubility in common deuterated solvents.
123

J IRAN CHEM SOC
Molar conductivity
templated [1 ? 1] cyclocondensation of 2,6-diacetylpyri-
dine and three new tetradentate amines. The complexes
have been characterized by some spectroscopic methods. In
addition, the solid state structure of [CdL1(CH3OH)](-
ClO4)2 shows the Cd(II) ion to be in a distorted pentagonal-
pyramidal geometry with a coordination number of six.
Results of this research showed that all prepared chemicals
have relatively antibacterial effects against the studied
bacterial strains, but the most effective ones are Cd(II)
complexes.
The molar conductivity measurements, recorded for 10-3
M
solutions at 25 °C of the Schiff base metal complexes in
DMSO. The molar conductivity was applied to help in the
investigation of the geometrical structures of the com-
plexes. Metal chelates have molar conductivity of
160–175 ohm-1 cm2 mol-1 indicating the 1:2 ionic nat-
ures in all cases [39]. These values are indicative of the
presence two outer sphere perchlorate anions and are in
agreement with the results obtained from the IR and X-ray
studies.
Acknowledgments We are grateful to the Faculty of Chemistry of
Bu-Ali Sina University and Ministry of Science, Research and
Technology of Iran, for financial support.
Antibacterial activity
The results of antibacterial studies are presented in
Table 3. The organisms used in this research were S.
aureus (ATCC 6633), B. cereus (ATCC 7064), C. xerosis
(ATCC 373) (Gram-positive bacterial strains), E. coli
(PTCC 10009), K. pneumoniae (MTCC 109), and P.vul-
garis (lio) (Gram-negative bacterial strains). The results
were recorded for each tested compound as the average
diameter of inhibition zones of bacterial growth sur-
rounding the well in millimeters. The compounds exhibited
variable antibacterial activity against the above tested
bacterial strains. Preliminary screening for all the com-
plexes was performed at fixed concentration of 10 mg/ml.
The results obtained were compared with standard
antibiotics: ciprofloxacin (for gram positive) and gentamicin
(for gram negative) bacterial strains. All the complexes
were found to be active on both types of bacterial strains.
On the basis of the data obtained for diameter of zone of
inhibition Cd(II) and Zn(II) complexes were found to be
very effective (Table 3). The results show that the anti-
bacterial activity of cadmium complexes is greater than
that of zinc complexes.
Appendix A
Supplementary data
CCDC 956695 contains the supplementary crystallo-
graphic data for [CdL1(CH3OH)] (ClO4)2. These data can
be obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (?44) 1223-336-033; or e-mail: deposit@
ccdc.cam.ac.uk.
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