Synthesis, characterization, and X-ray crystal structure of cobalt(III) complexes with a N2O2-donor Schiff base and ancillary ligands. Spectral, antibacterial activity, and electrochemical studies

Article abstract of DOI:10.1016/j.poly.2014.05.049

The synthesis and characterization of a series of cobalt(III) complexes of the general type trans-[Co(Me-salen)(L₂)]X, are described. The H ₂Me-salen = N,N′-bis(methylsalicylidene)-1,2-ethylenediimine) and the two ancillary ligands (

Full text of DOI:10.1016/j.poly.2014.05.049

Accepted Manuscript
Synthesis, characterization, and X-ray crystal structure of cobalt(III) complexes
with a N O -donor Schiff base and ancillary ligands. Spectral, antibacterial ac-
2
2
tivity, and electrochemical studies
Mehdi Salehi, Mehdi Amirnasr, Soraia Meghdadi, Kurt Mereiter, Hamid R.
Bijanzadeh, Ali Khaleghian
PII:
S0277-5387(14)00366-0
DOI:
http://dx.doi.org/10.1016/j.poly.2014.05.049
POLY 10764
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
10 January 2014
20 May 2014
Please cite this article as: M. Salehi, M. Amirnasr, S. Meghdadi, K. Mereiter, H.R. Bijanzadeh, A. Khaleghian,
Synthesis, characterization, and X-ray crystal structure of cobalt(III) complexes with a N O -donor Schiff base and
2
2
ancillary ligands. Spectral, antibacterial activity, and electrochemical studies, Polyhedron (2014), doi: http://
dx.doi.org/10.1016/j.poly.2014.05.049
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Synthesis, characterization, and X-ray crystal structure of cobalt(III)
complexes with a N O -donor Schiff base and ancillary ligands. Spectral,
2
2
antibacterial activity, and electrochemical studies.
a,
b
b
c
Mehdi Salehi *, Mehdi Amirnasr , Soraia Meghdadi , Kurt Mereiter ,
d
e
Hamid R. Bijanzadeh , Ali Khaleghian
a
Department of chemistry, Semnan University, Semnan, Iran
Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran.
b
c
Faculty of Chemistry, Vienna University of Technology, Getreidemarkt 9/164SC, A-1060
Vienna, Austria
d
Department of Chemistry, Tarbiat Modarres University, Tehran, P.O. Box 14155-4838, Iran
Semnan University of Medical Science, Faculty of Medicine, Biochemistry Department,
e
Semnan, Iran
Corresponding author:E-mail: msalehi@semnan.ac.ir (M. Salehi).
Abstract
The synthesis and characterization of a series of cobalt(III) complexes of the general type
trans-[Co(Me-salen)(L
,2-ethylenediimine) and the two ancillary ligands (L) in the trans positions are: benzylamine
bzlan) 1, 1-methylimidazole (1-MeIm) 2, morpholine (mrpln) 3, pyrolidine (prldn) 4, 3-
2 2
)]X, are described. The H Me-salen = N,N′-bis(methylsalicylidene)-
1
(
methylpyridine (3-Mepy) 5, 4-methylpyridine (4-Mepy) 6, pyridine (py) 7. These complexes
1
have been characterized by elemental analyses, IR, UV–Vis, and H NMR spectroscopy. The
III
III
crystal structures of trans-[Co (Me-salen)(bzlan)
2
]PF , 1, and trans-[Co (Me-salen)(3-
6
Mepy) ]PF , 5, have been determined by X-ray diffraction on solvated crystals. While the
2
6
Co(Me-salen) unit in 1 adopts a stepped non-planar conformation, an umbrella conformation
is assumed by the Co(Me-salen) unit in complex 5. The electrochemical reduction of these
complexes at a glassy carbon electrode in acetonitrile solution indicates that the first reduction
potential of Co(III/II) is irreversible, with the concomitant release of the axial ligands. Our
studies showed that the metal complexes exhibit moderate antibacterial activity, and their

antibacterial activity is higher than that of the ligand against both gram positive and gram
negative bacteria.
Keywords: Cobalt(III) complexes; N
2
O
2
Schiff base ligand; Crystal structure, Cyclic
voltammetry
1
. Introduction
Schiff bases offer a versatile and flexible series of ligands capable of binding to the
transition, non-transition, lanthanide and actinide metal ions to give complexes with suitable
properties for practical applications and/or theoretical studies [1-3]. A large number of
quadridentate Schiff bases and their cobalt complexes have been studied because of their
interesting and important properties such as their ability to reversibly bind oxygen [4-6], their
role in the elucidation of various aspects of bioinorganic chemistry [6,7-12], and their use in
catalyses for oxygenation and oxidation reactions of organic compounds [13-15]. The
coordination geometry of these complexes depends upon various factors including the size
and electronic configuration of the central metal ions, non-bonding interactions between
atoms in different ligand arms, and the inherent rigidity due to the presence of aromatic rings.
The present study, is an extension to our work [16–20]with the Schiff base ligand prepared by
2
the condensation of 2-hydroxyacetophenone with 1,2-ethylenediamine to give H Me-salen
(
Scheme 1).
O
2
OH
+
NH2 NH2
N
OH
N
HO
Scheme 1
The synthesis and spectroscopic characterization of seven new cobalt(III) complexes
III
(
Scheme 2) and the crystal structures of two representative compounds, trans-[Co (Me-
III
salen)(bzlan)
2
]PF
6
, 1, and trans-[Co (Me-salen)(3-Mepy)
2
]PF , 5, are reported. The results of
6
our cyclic voltammetry studies on these complexes are also presented and discussed.
2

Complex
L
Complex
(5)
L
l
g
i
k
j
NH2
h
(
1)
g
L
j
h
k
N
i
h
N
i
i
H
b
a
CH
3
j
g
N
N
Hc
h
g
(2)
Co
L
N
(6)
(7)
O
O
H
d
N
i
Hax
H
H
f
H
eq
H
e
ax
NH
Heq
h
h
g
g
(3)
H
eq
h
g
O
H
H
ax
N
eq
H
ax
H
N
g
(
4)
g
h
h
Scheme 2
2
. Experimental
2
.1. Materials and physical measurements
All solvents and chemicals were used as received, except for the axial ligands which were
distilled under reduced pressure prior to use. Elemental analyses were performed by using a
Perkin-Elmer 2400II CHNS-O elemental analyzer. UV-Vis spectra were recorded on a
JASCO V-570 spectrophotometer. Infrared spectra (KBr pellets) were obtained on a FT-IR
1
JASCO 680 plus spectrophotometer. H-NMR spectra were obtained on a BRUKER
AVANCE DR X500 (500 MHz) spectrometer. Proton chemical shift are reported in part per
million (ppm) relative to an internal standard of Me Si. The redox properties of the complexes
4
were studied by cyclic voltammetry. Cyclic voltammograms were recorded by using a SAMA
Research Analyzer M-500. Three electrodes were utilized in this system, a glassy carbon
working electrode, a platinum disk auxiliary electrode and Ag wire as reference electrode.
The glassy carbon working electrode was manually cleaned with 1 µm alumina polish prior to
each scan. Tetrabutylammonium hexafluorophosphate (TBAH) was used as supporting
2
electrolyte. Acetonitrile was dried over CaH . The solutions were deoxygenated by purging
3

with Ar for 5 min. All electrochemical potentials were calibrated against ferrocene-
ferrocenium couple under the same conditions [21].
2
.2. Synthesis of the ligand
Me-salen, N, N′-bis(methylsalicylidene)-1,2-ethylenediamine, was prepared as reported in
the literature [22] by the condensation of 1,2-ethylenediamine with 2-hydroxyacetophenone
1:2 molar ratio) in methanol at room temperature (Scheme 1), and was recrystallized from
H
2
(
-1
hot methanol. Yield: 80%. FT-IR (KBr, cm ): νmax 3383–3483 (m, br, OH), 1610 (s, C=N).
-
1
−1
UV–Vis: λmax (nm) (ε, L mol cm ) (CH
.3. General procedure for the synthesis of complexes
The complexes shown in Scheme 1 were synthesized as follows. To a stirring solution of
Co(CH COO) ·4H O (0.125 g, 0.5 mmol) in methanol (25 ml) was added an equimolar
amount of H
3
CN): 380 (960), 318 (7248), 252 (16600).
2
3
2
2
2
Me-salen (0.148 g, 0.5 mmol). The pink solution turned red immediately upon
II
formation the of [Co (Me-salen)] complex. The appropriate axial ligand (4 mmol) was then
added to this solution, and air was bubbled through the reaction mixture for about 3 h. To the
4 6 4
resulting dark red solution was then added 0.5 mmol of NH PF or NaClO and the mixture
was stirred for additional 5 minutes. The products were obtained as dark red microcrystalline
solids by slow evaporation of methanol at room temperature.
trans-[Co(Me-salen)(bzlan)
Yield: 70%. Anal. Calcd for C32
C, 54.00; H, 5.20; N, 7.70. FT-IR (KBr, cm ): νmax 3317-3261 (m, N-H), 1599 (s, C=N), 833
2
]PF
6
1: Recrystallization solvent: chloroform-propanol (2/1 v/v).
6 4 2
H36CoF N O
P (%): C, 53.94; H, 5.09; N, 7.86. Found (%):
-
1
–1
–1
(
(
s, PF6). UV–Vis: λmax (nm) (ε, L mol cm ) (CH
3
CN): 535 (356) 381 (10883), 255
87678).
trans-[Co(Me-salen)(1-MeIm)
2
]PF
6
2: Recrystallization solvent: methanol. Yield: 85%.
Anal. Calcd for C26 30CoF N O P (%): C, 47.14; H, 4.56; N, 12.68. Found: C, 47.05; H,
H
6 6 2
4

-1
4
.51; N, 12.57. FT-IR (KBr, cm ): νmax 1593 (s, C=N), 839 (s, PF6). UV–Vis: λmax (nm) (ε, L
–
1
–1
mol cm ) (CH
trans-[Co(Me-salen)(mrpln)
Calcd for C26 (%): C, 49.81; H, 5.79; N, 8.94. Found (%): C, 49.00; H, 6.00; N,
.40. FT-IR (KBr, cm ): νmax 3208 (m, N-H), 1597-1585 (s, C=N), 1085 (s, ClO ). UV–Vis:
3
CN): 507 (452), 380(7310), 260 (60885).
2
]ClO 3: Recrystallization solvent: methanol. Yield: 80%. Anal.
4
H
4 8
36ClCoN O
-1
8
4
–
1
–1
λ
max (nm) (ε, L mol cm ) (CH
]ClO
(%): C, 52.49; H, 6.10; N, 9.42. Found (%): C, 52.40; H, 6.30; N,
.50. FT-IR (KBr, cm ): νmax 3245 (m, N-H), 1590 (s, C=N), 1094 (s, ClO ). UV–Vis: λmax
3
CN): 555 (284), 383 (7301), 257 (46359).
trans-[Co(Me-salen)(prldn)
Calcd for C26
2
4
4: Recrystallization solvent: methanol. Yield: 55%. Anal.
H
4 6
36ClCoN O
-
1
9
4
–
1
–1
(
nm) (ε, L mol cm ) (CH
trans-[Co(Me-salen)(3-Mepy)
Calcd for C30
3
CN): 540 (288), 388 (9221), 255 (68167).
2
]PF 5: Recrystallization solvent: methanol. Yield: 65%. Anal.
6
H32CoF
6
N
4
O
2
P (%): C, 52.64; H, 4.71; N, 8.19. Found (%): C, 52.38; H, 4.65;
-1
–1
6
N, 8.12. FT-IR (KBr, cm ): νmax 1591 (s, C=N), 839 (s, PF ). UV–Vis: λmax (nm) (ε, L mol
–
1
cm ) (CH
trans-[Co(Me-salen)(4-Mepy)
Calcd for C30
3
CN): 527(351), 375(8467), 255(46642).
2
]PF 6: Recrystallization solvent: methanol. Yield: 55%. Anal.
6
H32CoF
6
N
4
O
2
P (%): C, 52.64; H, 4.71; N, 8.19. Found (%): C, 52.61; H, 4.74;
-1
–1
6
N, 8.15. FT-IR (KBr, cm ): νmax 1590 (s, C=N), 839 (s, PF ). UV–Vis: λmax (nm) (ε, L mol
–
1
cm ) (CH
trans-[Co(Me-salen)(py)
Calcd for C28
3
CN): 524(401), 376 (7833), 253 (50532).
2
]ClO 7: Recrystallization solvent: methanol. Yield: 60%. Anal.
4
H28ClCoN
4
O
6
(%): C, 55.05; H, 4.62; N, 9.17. Found (%): C, 54.83; H, 4.56; N,
-1
–1
9
4
.13. FT-IR (KBr, cm ): νmax 1590 (s, C=N), 1092 (s, ClO ). UV–Vis: λmax (nm) (ε, L mol
–
1
cm ) (CH
.4. Crystal structure determination
For X-ray data collection red crystals of 1 in the form of the solvate, 1·CHCl
3
CN): 530 (339), 376 (6235), 259 (52383).
2
3
, and dark red
crystals of 5 in the form of the disordered methanol solvate, 5·solv, were obtained by slow
5

evaporation of a chloroform/1-propanol (1/1 v/v) and methanol solution of the respective
complexes. X-ray data were collected on a Bruker Smart APEX CCD diffractometer with
graphite monochromated Mo Kα (λ = 0.71073 Å) radiation. Full spheres of reciprocal lattice
were scanned by 0.3° steps in omega with a crystal–to–detector distance of 5 cm. Cell
refinement and data reduction were performed with the help of the SAINT program [23].
Corrections for absorption was carried out with the multi-scan method and program SADABS
[
23]. The structures were solved with direct methods using SHELXS97 and structure
2
refinement on F was carried out with SHELXL97 program [24]. All non-hydrogen atoms
were refined anisotropically. Hydrogen atoms were inserted in idealized positions and refined
riding with the atoms to which they were bonded. For 1 a modest disorder of the CHCl
molecule was taken into account. In the case of 5 the PF group was disordered (two
alternative orientations in 0.769(5) / 0.231(5) proportion) and the solvent (CH OH) as well. In
3
6
3
this case the solvent was squeezed with program PLATON [25]. The crystallographic and
refinement data are summarized in Table 1.
Table 1. Crystal data and structure refinement for 1 and 5.
Empirical formula
C
33
H
37Cl
3
CoF
6
N
4
O
2
P
a
C
30
H32CoF
6
N
4
2
O P (5·solv)
(
3
1·CHCl )
Formula weight
Temperature(K)
831.92
100(2)
684.50
100(2)
Radiation, wavelength
Mo Kα, 0.71073
Mo Kα, 0.71073
Crystal system, space group
monoclinic, P2
10.9608(6)
25.1354(13)
13.3639(7)
104.532(1)
3564.0(3)
4
1
/n
Monoclinic, P2
15.4352(8)
12.9444(7)
17.3965(10)
113.173(1)
3195.4(3)
4
1
/c
a (Å)
b (Å)
c (Å)
β (°)
3
Volume (Å )
Z
-
3
D
calc (g cm )
1.550
1.428
3
Crystal colour and size (mm )
red, 0.60 × 0.40 × 0.30
0.804
dark red 0.60 × 0.53 × 0.30
0.656
-
1
µ (mm )
F (000)
1704
1408
θ Range (°)
Index range
2.26 - 30.02
−15 ≤ h ≤ 15
2.55 - 30.10
-21≤ h ≤ 21
-18 ≤ k ≤ 18
−35 ≤ k ≤ 35
6

−
18 ≤ l ≤ 18
-24 ≤ l ≤ 24
45615
Reflection collected
46733
Independent reflections
Reflections with F >2σ
10347 [Rint = 0.0285]
8656
9390[Rint = 0.0218]
8279
2
Absorption correction
Max. and min. transmission
multi-scan
0.78 and 0.63
multi-scan
0.82 and 0.67
Data/ restraints/ parameters
10347/ 0/ 467
1.087
9390/ 342/ 425
1.092
2
Goodness-of-fit on F
Final R indices [F > 2σ (F )]
R indices (all data)
2
2
R
1
= 0.0418, wR
2
2
= 0.0961 R
= 0.0355, wR
= 0.0950
= 0.0989
1
2
2
R
1
= 0.0542, wR
= 0.1030 R
1
= 0.0416, wR
-
3
Largest difference peak, hole (e Å ) 0.95 and -0.88
0.73 and -0.88
2
.5 Biological studies
2
.5.1. Bacterial strains.
Metal complexes and ligands were individually tested against a penal of microorganisms
Gram negative and Gram positive), namely Bacillus subtilis (B. subtilis; Gram positive,
(
ATCC 6633); Staphylococcusaureus (S.aureus; Gram positive, ATCC 25923), Salmonella
Typhi (S. Typhi; Gram negative, ATCC 19430) and Escherichiacoli (E.coli;Gram negative,
ATCC 25922), The organisms were purchased from Iranian Research Organization for
Science and Technology (IROST).
2
.5.2. Minimum inhibitory concentrations (MICs)
In vitro antibacterial activities of the complexes of the chemicals were carried out by the broth
twofold dilution method as a quantitative assay [26]. Briefly, serial diluted chemical
compounds in the range of 1- 6.2 µg/mL were added to a final inoculum of approximately
6
1
.5×10 organisms per mL in log-phase growth. The cultures were incubated on a rotary
shaker at 37°C for 24 h. Minimum inhibition concentration MIC (µg/mL) which is defined as the
lowest concentration of the sample which inhibits bacterial growth was determined in sterile plates
compared to the drug-free control wells. All the tests were performed in duplicates on
different days to measure the reproducibility of the tests.
2
.5.3 Minimal bactericidal concentration (MBC)
7

To measure the minimal bactericidal concentration (MBC), 100 µL volumes of all clear (no
growth) tubes from a dilution MIC test was spread on to separate agar plates and incubated at
3
7 °C for 24 h. For determination of the MBC (mg/ml) as the lowest concentration that did
not grow on nutrient agar plate [27].
.5.4 Disc diffusion assay.
2
Antibacterial activity of the ligands and metal complexes were carried out using the disc
diffusion method as a qualitative assay, described by Bauer et al. [28]. Four to five colonies of
each organism were inoculated into 4 mL of broth and incubated for 4-6 h at 37° C. Bacterial
suspension’s turbidity was compared and equalized with the Mac Farland 0.5 standard [29].
Bacteria were cultured on agar plates using sterile absorbent cotton swabs. Each organism
was tested in duplicate on different days to measure the reproducibility of the test.
Kanamycine (10 µg per disc) and Chloramphenicol (10 µg per disc), purchased from Padtan
Teb Company (Iran), were used as reference antibacterial agents. A set of assay tubes
containing only inoculated medium was kept as negative control and likewise solvent controls
were also done simultaneously. All assays were performed in duplicate.
3
. Result and discussion
3
.1. Synthesis
All cobalt(III) complexes were prepared by the direct reaction of the corresponding ligand,
Me-salen, and Co(CH COO) ·4H O in the presence of the appropriate axial ligand under
H
2
3
2
2
aerobic conditions (Scheme 2). In the preparation of complexes (1-7), a vigorous stream of air
II
was passed through a solution of Co (Me-salen) in methanol to which was gradually added
excess ancillary ligand. The air oxidation was continued for a period of 3 h, during which the
color changed from red to red brown. Red to dark red crystals of these complexes was
obtained in good yield (50-85%).
3
.2. Spectroscopic studies
8

1
The structures were identified by infrared, UV–Visible, Elemental Analyses, and H-NMR
spectra (Table 2 and Table 3). A comparative study of the FT-IR spectral data of the reported
complexes with those of the uncomplexed ligands gives meaningful information regarding the
2
bonding sites of the ligand molecules. H Me-salen shows a broad band characteristic of the
−1
OH group in the 3383-3483 cm regions. The absence of this band in the FT-IR spectra of
the complexes is indicative of the fact that the tetradentate ligand is coordinated as a dianion.
-
1
The ν(C=N) band appearing at 1610 cm in H
2
Me-salen is shifted to lower frequencies by
-1
1
1–25 cm in the corresponding cobalt complexes, indicating a decrease in the C=N bond
order due to the coordination of the imine nitrogen to the metal and back bonding from the
Co(III) metal to the π* orbital of azomethine group [18].
1
The H-NMR spectral measurements were performed in CDCl
3
solution. The main parameters
1
of H-NMR spectroscopic data of seven cobalt complexes are given in the Table 3. The
observed signals in Me-salen ligand at 3.99 (singlet), 2.39 (singlet) and 6.5–7.52 ppm are
3
assigned to aliphatic protons of the ethylene chelate ring, the two CH groups on Me-salen
and the aromatic protons of the phenyl rings of the equatorial ligand, respectively. The signal
1
appearing as a singlet at 3.94–4.45 ppm in the H NMR spectrum of these complexes is
assigned to the two CH
3
groups on Me-salen and indicates that the equatorial coordination
protons.
sites are occupied by Me-salen ligand leading to the magnetically equivalent CH
3
The signals due to the aliphatic protons of the ethylene chelate ring appear at δ = 2.80–2.96
ppm. The aromatic protons of the phenyl rings in the coordinated Schiff base ligand appear in
the appropriate region at 6.49-7.7 ppm.
The electronic absorption spectrum of H Me-salen in acetonitrile consists of two relatively
2
intense bands centered at 252 and 318 nm, assigned to the πꢀπ* transitions of the benzene
ring of salicylaldehyde and the azomethine group respectively. A third band at 380 nm,
corresponding to nꢀπ*, disappears from the UV-Vis spectra of the cobalt complexes upon
9

coordination of the ligand [30, 31]. The intraligand πꢀπ* transition of the azomethine group
is red shifted by about 70 nm in the corresponding cobalt complexes and appears in the range
of 375 to 388 nm [32]. The first dꢀd transition in different Me-salen cobalt complexes (1-7)
appears as a shoulder in the range of 510 to 555 nm (ε = 283-434). This transition is well
correlated with the basicity of the axial ligands and the reduction potentials of the
corresponding Co(III) complexes.
Table 2. FT-IR, analytical data and UV-Vis for complexes.
a
Complex
Selected IR
Elemental analysis
Anal.Calc. (Found)
C: 53.94 (54.00); H: 5.09 (5.20); N:
UV-Vis: λmax (nm)
−1
−1 b
υ
(C=N), υ(P-F) or υ (Cl-O)
(ε, M cm )
535 (356), 381 (10883),
255 (87678)
507 (452), 380(7310),
260 (60885)
555 (284), 383 (7301),
257 (46359)
540 (288), 388 (9221),
255 (68167)
527(351), 375(8467),
255(46642)
524(401), 376 (7833),
253 (50532)
530 (339), 376 (6235),
259 (52383)
1
2
3
4
5
6
1599, 833
1593, 839
7
.86 (7.70)
C: 47.14 (47.05); H: 4.56 (4.51); N:
2.68 (12.57)
C: 49.81 (49.00); H: 5.79 (6.00); N:
.94 (8.40)
C: 52.49 (52.40); H: 6.10 (6.30); N:
.42 (9.50)
C: 52.64 (52.38); H: 4.71 (4.65); N:
.19 (8.12)
C: 52.64 (52.61); H: 4.71 (4.74); N:
.19 (8.15)
C: 55.05 (54.83); H: 4.62 (4.56); N:
1
1
597-1585, 1085
8
1590, 1094
1591, 839
1590, 839
1590, 1092
9
8
8
7
9
.17 (9.13)
a
b
KBr pellets, Solvent is acetonitrile.
1
III
Table 3. HNMR data for trans-[Co (Me-salen)(L)
2
)]X at 298 K.
L
Bzlan
H
2.75
(br s)
a
H
3.94
(br s) (m)
(4H) (2H)
b
H
6.70
d
H
f
H
.86
br dd)
4H,He,f )
e
H
7.70
(br d)
( 2H)
c
Other peaks
3.23 ( m, 4H, H )
g
7.20 (m, 6H, Hl,h,j
7.33 (m,4H, Hi,k
6
(
1)
)
(
(
6H)
)
(
2
.73 (br s, 4H)
1
-MeIm 2.80
2) (br s)
6H)
4.10
(br s)
(4H)
6.52-7.47 (14H,Hc,d,e,f,g,i,j)
(
h
3.57 (br s, 6H, H )
(
Mrpln
3)
2.96
(br s)
4.45
(brs) (br t)
(4H) (2H)
6.70
7.39
(br d)
(2H)
7.32
(br t)
(2H)
7.65
(br d)
(2H)
2.12 (br m, 4H, Hg,ax
2.27 (br d, 4H, Hg,eq
3.25 (m, 4H, Hh,ax
)
)
(
(
6H)
)
3
.48 (br dd, 4H, Hh,eq
)
2
.54 (br t, 2H)
Prldn
4)
2.87
(br s)
4.27
(s)
6.63
(br t)
7.36
(br)
7.27
(br m)
7.59
(br d)
1,48 (br s, 8H, H
1.95, 2.32 (br m, 8H,
h
)
(
(
6H)
(4H) (2H)
(2H)
(2H)
(2H)
g
H )
1
0

3
.01 (m, 2H,)
3
4
-Mepy 2.84
5) (br s)
6H)
4.17
(br s) (br t)
(4H) (2H)
6.49
7.28
(br d)
(2H)
7.14
(br td)
2H)
7.38
(br dd)
(2H)
2.18 (br s, 6H, H
7.10 (br t, 2H, H
i
7.40 (br d, 2H, H )
j
)
(
h
)
(
7
8
.88 (br d, 2H, H
.01 (s, 2H, H
g
)
k
)
-Mepy 2.82
6) (br s)
6H)
4.15
(br s) (br t)
(4H) (2H)
6.50
7.28
(br m)
(2H)
7.15
(br t)
(2H)
7.38
(br dd)
(2H)
2.28 (br s, 6H, H
7.00 (br d, 4H, H
g
7.95 (br d, 4H, H )
i
)
(
h
)
(
Py
7)
2.87
(br s)
4.25
(br s) (br d)
(4H) (2H)
6.50
7.29
(br dd)
(2H)
7.16
(br td)
(2H)
7.38
(br dd)
(2H)
7.63 (br t, 2H, H
8.22 (br d, 4H, H
7.23 (br t, 4H, H )
i
)
g
)
(
(
6H)
h
Me-salen
2.39
3.99
(br s) (br td)
4H) (2H)
6.79
6.93
(br d)
(2H)
7.28
(br td)
(2H)
7.52
(br dd)
(2H)
15.80
(br s)
(2H)
(
s)
(
6H)
3
.3. Crystal structures of complexes (1) and (5)
III
2 6
.3. 1. Trans-[Co (Me-salen)(bzlan) ]PF (1)
3
The structure of complex 1 consists of a mononuclear cobalt(III) unit, depicted in Fig 1.
Selected bond lengths and angles are given in Table 4.
Table 4. Selected bond lengths [Å] and angles [°] for 1 and 5.
1
5
Bond lengths
Co1-O1
Co1-O2
Co1-N1
Co1-N2
1.8782(13)
1.8783(13)
1.8992(15)
1.9051(15)
1.9762(15)
1.9751(15)
Co1-O1
Co1-O2
Co1-N1
Co1-N2
Co1-N3
Co1-N4
1.8867(11)
1.8858(11)
1.8968(14)
1.8927(14)
1.9529(14)
1.9735(14)
Co1-N3
Co1-N4
Bond angles
O1-Co1-O2
O1-Co1-N1
O2-Co1-N1
O1-Co1-N2
O2-Co1-N2
N1-Co1-N2
O1-Co1-N4
O2-Co1-N4
85.48(6)
93.48(6)
178.75(6)
178.40(7)
93.80(6)
87.25(7)
85.71(7)
86.81(6)
O2-Co1-O1
O1-Co1-N1
O2-Co1-N1
O1-Co1-N2
O2-Co1-N2
N2-Co1-N1
O1-Co1-N4
O2-Co1-N4
89.88(5)
91.43(5)
178.55(6)
176.15(6)
92.82(5)
85.91(6)
86.55(5)
88.65(6)
1
1

N1-Co1-N4
N2-Co1-N4
O1-Co1-N3
O2-Co1-N3
N1-Co1-N3
N2-Co1-N3
N4-Co1-N3
93.82(6)
92.82(7)
88.17(6)
88.09(6)
91.18(6)
93.24(6)
172.32(7)
N1-Co1-N4
N2-Co1-N4
O1-Co1-N3
O2-Co1-N3
N1-Co1-N3
N2-Co1-N3
N3-Co1-N4
92.05(6)
90.76(6)
90.95(5)
88.84(5)
90.51(6)
91.85(6)
176.46(6)
The Me-Salen ligand shows a transoid conformation in complex 1. The average values of the
Co–O and Co-N bond lengths in the equatorial ligand plane are 1.8783(13) and 1.9022(15) Å,
respectively. The distances between the cobalt atom and the two axial nitrogen donor atoms
differ only slightly (1.9762(15) and 1.9751(15) Å) and compare well with the Co–N distances
found in cobalt-ammine complexes, e.g. [Co(NH
acacen)(NH ]BPh (Co–N = 1.955 Å) [33, 7]. The Co–Neq is shorter than Co–Nax by about
.073 Å in complex 1 due to the presence of π-backbonding in Co–Nimine of the equatorial
ligand. The difference is 0.069 Å in [Co(3-Cl-acacen)(NH ]BPh (Co–N = 1.955 Å). The
solid state compound contains a CHCl molecule which shows a moderate underoccupancy
and subordinate disorder (two solvent peaks near Cl(3)). The two NH groups show weak
hydrogen bonding, for each one NH group only one H is hydrogen bonded with a nearby PF
3
)
6
]Cl
3
(Co–N = 1.963 Å) and [Co(3-Cl-
3
)
2
4
0
3
)
2
4
3
2
2
6
octahedron (Table 5), while each second hydrogen exhibits no significant H-bond interaction.
There is also some C-H---F interactions, which are not included in the H-bond compilation.
One of these C-H---F interactions is between the CHCl carbon C(1s) and F(1).
3
III
Table 5. Hydrogen bonds for trans-Co (Me-salen)(bzlan)
2
]PF
6
(1·CHCl ).
3
D-H···A
d(D-H) (Å) d(H···A) (Å) d(D···A) (Å)
<(DHA) (°)
174
160
130
139
113
147
112
a
N3-H3B...F3
N4-H4B...F6
C9-H9A...F4
C9-H9B...F2
C19-H19A...O2
0.92
0.92
0.99
0.99
0.99
0.95
0.99
2.27
2.49
2.49
2.41
2.58
2.54
2.56
3.189(2)
3.374(2)
3.218(3)
3.226(3)
3.098(3)
3.373(3)
3.068(3)
a
a
C21-H21...F1
C26-H26A...O1
Symmetry transformation used to generate equivalent atoms:
a= -1/2+x,1/2-y,-1/2+z
III
.3. 2. Trans-[Co (Me-salen)(3-Mepy) ]PF (5)
2 6
3
1
2

As described in the experimental section, the solid state form of complex 5 was a disordered
solvate 5·solv with an unknown solvent content obtained from methanol. The structure
contains a mononuclear cobalt(III) unit, depicted in Fig 2. Selected bond lengths and angles
are given in Table 4. The Me-salen ligand shows a cisoid conformation in complex 5. There
are two 3-methylpyridine molecules attached to the metal atom to fill its axial sites with a
perpendicular disposition. The two phenolic oxygen atoms and the two imine nitrogen atoms
are located in cis positions. The average Co–Neq distance (1.8948 Å) is considerably longer
III
than that of [Co(salen)(py)
2
]BPh
4
(1.888 Å) and shorter than that of [Co (Me-salpn)(py)
2
]PF
6
(
1.963 Å) [20]. The N(1)–Co–N(2) angle (85.91(6)˚) is smaller than the corresponding angle
III
in the related [Co (Me-salpn)(py)
2 6
]PF complex (95.69(7)˚) and slightly larger than in the
related salen complex (85.5(2)˚) [34].
3
.4. Electrochemical studies
The cyclic voltammetric studies of trans-[Co(Me-salen)(L)
conducted at 25 ˚C under an argon atmosphere using acetonitrile solutions containing 0.1 mol
2
]X complexes (1-7) were
-
3
-3
-3
dm TBAH as supporting electrolyte and complex concentrations of about 4 × 10 mol dm .
The reduction potential data of cobalt complexes are summarized in Table 6.
Table 6. Redox potentials of cobalt complexes of Me-salen in acetonitrile at 298 K.
Complex
E
–
pc1
E
pa1
E
pc2
pK
b
III
[
(
[
Co (Me-salen)(bzlan)
with excess bzlan)
Co (Me-salen)(bzlan)
2
]PF
6
6
[
35]
–
0.629
0.791 –0.073
0.549 0.001
–1.468
5.18
0.791
III
2
]PF
–
–
–
–
–
–
–
–1.468
–1.470
–1.467
–1.472
–1.466
–1.466
–1.472
5.18
III
[
[
[
[
[
[
Co (Me-salen)(py)
2
]ClO
4
[36]
8.77
5.51
2.69
III
Co (Me-salen)(mrpln)
2
]ClO
]ClO
]PF
]PF
]PF
4
[35]
35]
[36]
0.551 –0.004
0.723 –0.147
III
[
Co (Me-salen)(prldn)
2
4
Co(Me-salen)(3-Mepy)
2
6
0.580
0.118
8.3
III
[
36]
Co (Me-salen)(4-Mepy)
2
6
0.606 –0.013
0.848 –0.087
8.01
7.05
III
Co (Me-salen)(MeIm)
2
6
[37]
1
3

The ligand is electro-inactive over the range of +0.7 to –1.8 V. The cyclic voltammogrm of 1
is presented in Fig 3. The first irreversible reduction at the -0.791 V is attributed to the
following reaction:
III
+
II
trans-[Co (Me-salen)(bzlan)
2
] + e
trans-[Co (Me-salen)] + 2 bzlan
2
The addition of an electron to the antibonding dz orbital leads to the loss of the axial ligand
even in weakly coordinating solvents [7, 18, 20]. The results show that the nature of the axial
ligand has a strong influence on the anodic peak potentials (Epa) for the first reduction process
III
–
II
Co + e → Co (differing by almost 299 mV in going from MeIm to py). This is, as
expected, due to the fact that the electron affinity of cobalt(III) in these complexes increases
with increasing pK
reduction potentials shift to more positive values for axial ligands of lower basicity. In fact,
there is a good correlation between the pK of the axial ligand, the position of the first d-d
b
(decreasing the basicity) of the axial ligand; therefore, the corresponding
b
transition in the electronic spectra, and the first cathodic potential (Epc, Co(III/II)) of the
complexes. These results are in agreement with those obtained for similar Co(III) complexes
reported in the literature [7, 18, 20].
The second electrochemically quasi-reversible reduction couple observed at –1.458 V (∆E
=
83 mV) is related to the following one-electron transfer process:
II
I
-
trans-[Co (Me-salen)] + e
trans-[Co (Me-salen)]
The cyclic voltammograms of 2-7 in the absence of axial ligands show features similar to
those observed for complex 1 (Table 6).
The reversibility of the first redox process in many Co(III) Schiff base complexes is
greatly influenced by many controlling factors including the concentration of the ancillary
III
II
ligands [7, 18]. In an attempt to examine the reversibility of (Co /Co ) reduction process, the
III
electrochemistry of trans-[Co (Me-salen)(bzlan)
2
]PF
6
was carried out in the presence of
III
excess bzlan. Fig. 4 shows the cyclic voltammogram of trans-[Co (Me-salen)(bzlan)
2
]PF
6
(1)
1
4

at different bzlan concentrations. In the absence of additional bzlan the first reduction process,
III
II
(
Co /Co ), is electrochemically irreversible (ꢀE = 718 mV). Upon addition of excess bzlan
ligands, the oxidation wave of Co(II) is shifted towards more negative potentials by 0.556 V
Fig. 6) (ꢀE = 162 mV). Apparently, the electrode process is preceded by a reversible
chemical reaction (eq. (1)), leading to the formation of the six-coordinated adduct.
(
II
III
+
–
[
Co (Me-salen)] + excess bzlan ligand
[Co (Me-salen)(L)
2
] + e
(1)
The redox behavior of the Co(II/I) couple is not significantly influenced by the addition of
excess axial ligand and can be regarded as approximately independent.
3
.5 Antibacterial properties
The in vitro antibacterial activities of the Schiff base and the complexes were tested against
two Gram-positive and two Gram-negative bacteria by disc diffusion methods using
Kanamycine and Chloramphenicol as reference standards, according to the Clinical and
Laboratory Standards Institute (CLSI) interpretive criteria [38]. Table 7 and Table 8 show
the MICs and the MBCs for the studied ligand, complexes and two standard compounds,
against the bacterial strains, respectively.
Table 7. MIC of synthesized compounds against growth of bacteria (µg/ml).
Compound
Gram Negative Bacteria
Gram Positive
S. aureus
6200
3800
6000
2500
3500
3500
4200
Bacteria
B.subtilis
5000
3500
4800
1000
3000
3000
3000
E.coli
6000
4000
5000
3000
4000
4000
4200
S. typhi
6500
4200
5200
2000
2000
4500
4000
Ligand
Complex 1
Complex 2
Complex 3
Complex 4
Complex 5
Complex 6
Complex 7
Kanamycine
Chloramphenicol
5000
3.8
5.3
3500
3.2
4
3000
3
3.5
3000
3.6
4
1
5

Table 8. MBC of synthesized compounds against growth of bacteria (µg/ml).
Compound
Gram Negative Bacteria
Gram Positive
S. aureus
7000
Bacteria
B.subtilis
5200
E.coli
8000
5000
6200
5000
5000
5200
5000
S. typhi
8000
5000
5500
3000
2500
5000
4000
Ligand
Complex 1
Complex 2
Complex 3
Complex 4
Complex 5
Complex 6
4200
6200
3000
4500
4000
5000
4000
5200
2000
3000
5000
4000
Complex 7
Kanamycine
Chloramphenicol
5500
3.8
5.3
5000
3.2
4
4000
3
3.5
3500
3.6
4
A comparative study of the ligand and complexes indicates that complex 3 exhibits higher
antimicrobial activities against the Gram negative and Gram positive bacteria than the free
ligand, thus the axial ligands may be useful in management of such infections. The enhanced
antibacterial activity of this axial ligand may be due to the presence of oxygene atom, and so,
the complex of 3 exhibits antimicrobial activities against the Salmonella Typhi and Bacillus
subtilis. In general, the antibacterial results show that the majority of the metal complexes
were more active than their respective Schiff bases. It is evident from the MBC data that the
antimicrobial activities of complex 6 against Gram negative and Gram-positive bacteria
tested, are higher as compared with ligand and other complexes. This enhancement in the
activity is suggested to be possibly due to an efficient diffusion of the metal complexes into
the bacterial cell or interaction with the bacterial cell walls [39].
4
. Conclusion
We have prepared seven Co(III) complexes of Me-salen with two ancillary ligands in the
trans-positions. The main features of the coordination and redox chemistry of the trans-
III
+
[
Co (Me-salen)(L) ] complexes have been illustrated by the combination of spectral,
2
structural, and electrochemical measurements. Electrochemical studies have revealed that the
first reduction process Co(III/II) is irreversible and strongly influenced by the nature of the
axial ligands. This process becomes quasi-reversible upon the addition of excess axial ligands.
1
6

The Co(II)/Co(I) couple is reversible and insensitive to moderate concentrations of axial
III
ligands. The antimicrobial data exhibited that most of Co complexes are more active than
those of corresponding free ligand against bacterial.
Acknowledgements
Partial support of this work by the Isfahan University of Technology Research Council is
gratefully acknowledged.
Supplementary material unjustified
Detailed crystallographic data have been deposited in CIF format. CCDC 715071 (1) and
7
15072 (5) contains the supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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1
9

Figure captions
6
Fig. 1. Structural drawing of complex 1 showing 50% ellipsoids. Hydrogen atoms, PF anion
3
and CHCl solvent molecule are omitted for clarity.
Fig. 2. Structural drawing of complex 5 showing 50% ellipsoids. Hydrogen atoms and PF
6
anion are omitted for clarity.
III
Fig. 3. Cyclic voltammogram corresponding to trans-[Co (Me-salen)(bzlan)
2
]PF
6
in
-
1
-3
acetonitrile solution stored for 48 h. at 293 K. Scan rate: 100 mV s . c = 4.0 × 10 M.
III
+
Fig. 4. Cyclic voltammograms of trans-[Co (Me-salen)(bzlan)
2
] at different benzylamine
concentrations ratios (0, a), (10, b), (30, c), (50, d), (70, e), (80, f), (90, g), (100, h), in
-
1
acetonitrile containing 0.1 M TBAH as supporting electrolyte at 298 K. Scan rate 100 mV s ,
-3
c = 4.0 × 10 M.
2
0

Fig 1
Fig 2
2
1

Fig 3
Fig 4
2
2

Graphical abstract