Synthesis, spectroscopic characterization, and antimicrobial activity of cobalt(II) complexes of acetone-N(4)-phenylsemicarbazone: Crystal structure of [Co(HL)2(MeOH)2](NO3)2

Article abstract of DOI:10.1007/s11243-011-9485-z

Four Co(II) complexes, [Co(HL)₂](OAc)₂, [Co(HL) ₂Cl₂], [Co(HL)₂(MeOH)₂](NO ₃)₂, and [Co₂(HL)₄(SO ₄)₂] (HL = acetone-N(4)-ph

Full text of DOI:10.1007/s11243-011-9485-z

Transition Met Chem (2011) 36:417–424
DOI 10.1007/s11243-011-9485-z
Synthesis, spectroscopic characterization, and antimicrobial
activity of cobalt(II) complexes of acetone-N(4)-
phenylsemicarbazone: crystal structure
of [Co(HL) (MeOH) ](NO )
3 2
2
2
V. L. Siji
S. Suma
•
M. R. Sudarsanakumar
•
Received: 19 December 2010 / Accepted: 17 March 2011 / Published online: 9 April 2011
Ó Springer Science+Business Media B.V. 2011
Abstract Four Co(II) complexes, [Co(HL) ](OAc) ,
2
antimicrobial [1], anticonvulsant [2], and antitumor [3]
2
[
(
Co(HL) Cl ], [Co(HL) (MeOH) ](NO ) , and [Co (HL) -
2
activities. A significant advantage of semicarbazone deriv-
atives over their thiosemicarbazone analogues seems to be
their lower toxicity [4]. The synthesis and reactivity of
cobalt complexes of Schiff base ligands have long attracted
the attention of inorganic chemists [5]. The cobalt com-
plexes of tetradentate Schiff base ligands have been
extensively used to mimic cobalamin (B ) coenzymes [6],
2
2
2
2
3 2
4
SO ) ] (HL = acetone-N(4)-phenylsemicarbazone) were
4
2
synthesized and characterized by physicochemical and
spectroscopic methods. The magnetic susceptibility mea-
surements indicate that the complexes are paramagnetic
with three unpaired electrons. In all the complexes, the
semicarbazone is coordinated as a neutral bidentate ligand.
The structure of [Co(HL) (MeOH) ](NO ) was confirmed
1
2
and as dioxygen carriers and oxygen activators [7]. They
are also used for enantioselective reduction [8] and as
antimicrobial agents [9]. Some cobalt complexes of pyri-
dine-2-carbaldehyde thiosemicarbazone have been found to
show cytotoxic activity [10], while the conformationally
rigid cobalt-based molecular square of tetra(2-pyridyl)-
thiosemicarbazone revealed interesting magnetic properties
[11].
2
2
3 2
by single crystal X-ray crystallography. The ligand is
neutral and bidentate, being coordinated to the cobalt atom
through the carbonyl oxygen and the azomethine nitrogen.
Intermolecular hydrogen bonding and C–Hꢀꢀꢀp interactions
combine to stabilize the crystal structure. The ligand and
its two complexes [Co(HL) Cl ] and [Co(HL) (MeOH) ]-
2
2
2
2
(
NO ) were screened for their antibacterial and antifungal
3 2
activities using disk diffusion methods.
The coordination possibilities derived from the many
potential donor atoms in the semicarbazones are increased
if the substituents include additional donor atoms and
they can also be modified by placing substituents on the
backbone donor atoms. The p-delocalization and the
configurational flexibility of their molecular chains can
give rise to a great variety of coordination modes [12].
These classes of compounds usually react with metals to
give complexes in which the semicarbazones behave as
chelating ligands. Semicarbazones exist in two tautomeric
forms, keto and enol [13], and can coordinate to the metal
either as a neutral ligand or as a deprotonated anion.
Currently, there are only very few reports on cobalt
complexes of N(4)-substituted semicarbazones [14].
Keeping in view the above points and with the aim of
elucidating the coordination behavior of acetone-N(4)-
phenylsemicarbazone (HL), its Co(II) complexes were
synthesized and characterized. The keto form of HL is
given in Scheme 1.
Introduction
Semicarbazones are very promising compounds from the
view of point of coordination chemistry because of their
interesting range of pharmacological properties including
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-011-9485-z) contains supplementary
material, which is available to authorized users.
V. L. Siji ꢀ M. R. Sudarsanakumar (&)
Department of Chemistry, Mahatma Gandhi College,
Thiruvananthapuram, Kerala 695 004, India
e-mail: sudarsanmr@gmail.com
S. Suma
Department of Chemistry, S.N. College, Varkala,
Kerala 695 145, India
123

4
18
Transition Met Chem (2011) 36:417–424
O
fungal spores onto Rose Bengal Agar (RBA) plates. The
compounds were dissolved in chloroform to a final con-
centration of 0.1%. The petri plates were incubated for
24 h for bacterial cultures and 76 h for the fungal cultures.
The activity of each test compound was assessed by mea-
suring the diameter of the inhibition zone in millimeters.
Test substances that produced a zone of inhibition of
diameter 9 mm or more were regarded as positive, while in
those cases where the diameter was below 9 mm, the
sample was considered to have no antimicrobial activity.
7
9
6
N
1
4
N
H
2
3
8
N
H
5
10
Scheme 1
Experimental
Acetone (Merck), N(4)-phenylsemicarbazide (Sigma–Aldrich),
Co(OAc) ꢀ 4H O (Merck), CoCl ꢀ 6H O (Merck), Co-
2
2
2
2
(
NO ) ꢀ 6H O (Merck), and CoSO ꢀ 6H O (Merck) were
3
2
2
4
2
of analar grade and used as received. Methanol (Merck) was
used without further purification. All the test microorgan-
isms were obtained from the Microbial Type Culture Col-
lection and Gene Bank (MTCC), Institute of Microbial
Technology, Chandigarh, India.
Physical measurements
Elemental analyses were carried out using a Vario EL III
CHN analyzer. Infrared spectra were recorded on a
Thermo Nicolet Avator 370 spectrometer in the range
-
1
4
,000–400 cm using KBr pellets. Electronic spectra were
Synthesis of acetone-N(4)-phenylsemicarbazone (HL)
recorded on a Cary 5000 Version 1.09 UV–VIS–NIR
spectrophotometer in the range 900–200 nm using solutions
in methanol. The room temperature magnetic susceptibility
measurements were taken in the polycrystalline state on a
Vibrating Sample Magnetometer (VSM) at 5.0 kOe field
strength. The molar conductances of the complexes in
The synthesis of acetone-N(4)-phenylsemicarbazone (HL)
has been published by us earlier [15]. An equimolar mixture
of N(4)-phenylsemicarbazide (0.151 g, 1 mmol) and ace-
tone (1 mmol) was heated at reflux for 3 h in methanol
(
30 mL) in the presence of three to four drops of dilute acetic
-
3
methanol (10 M) solutions were measured at 298 K with
a Systronics model 303 direct reading conductivity meter.
acid. On slow evaporation, colorless crystals of HL separated
out. These were filtered off, washed with ether, and dried
over P O in vacuo, then recrystallized from methanol.
4
10
1
HL: Yield: 80%; M.P.: 152–154 °C. H NMR (d ppm in
X-ray crystallography
CDCl ): 8.04 (1H, s), 8.21 (1H, s), 7.02–7.52 (5H, m), 2.03
1
3
3
(
3H, s), 1.90 (3H, s), C NMR (d ppm in CDCl ): C(4),
3
Single-crystal X-ray diffraction experiments for brown
crystals of [Co(HL) (MeOH) ](NO ) were performed on
1
53.69; C(2), 147.72; C(5), 138.25; C(7) & C(9), 128.90;
2
2
3 2
C(8), 123.10; C(6) & C(10), 119.36, C(3), 25.28; C(1), 16.52.
Synthesis of the complexes
a Bruker AXS Kappa Apex2 CCD diffractometer with
˚
graphite-monochromated Mo Ka (k = 0.71073 A) radia-
tion. A crystal with dimensions 0.25 9 0.20 9 0.20 mm
3
was selected and found to be orthorhombic space group
Pbca. The crystallographic data and structure refinement
parameters are given in Table 1. The unit cell dimensions
and intensity data were recorded at 293 K. The program
XPREP in SAINT-Plus was used for data reduction and
APEX2/SAINT-Plus for cell refinement [17]. The structure
was solved by direct methods using SHELXS-97 and
A solution of HL (0.382 g, 2 mmol) in methanol (20 mL)
was treated with a methanolic solution of 1 mmol quanti-
ties of Co(OAc) ꢀ 4H O (0.249 g) for [Co(HL) ](OAc) ,
2
2
2
2
CoCl ꢀ 6H O (0.238 g) for [Co(HL) Cl ], Co(NO ) ꢀ
2
2
2
2
3 2
6
H O (0.291 g) for [Co(HL) (MeOH) ](NO ) , and CoSO ꢀ
2
2
2
3 2
4
6H O (0.281 g) for [Co (HL) (SO ) ]. The solution was
4 2
2
2
4
heated at reflux for 5 h. The resulting solution was allowed
to stand at room temperature and upon slow evaporation
crystals separated out, which were collected, washed with
methanol and finally with ether, and dried over P O in
2
refined by full-matrix least squares on F using SHELXL-
97 [18]. Anisotropic displacement parameters were refined
for non-hydrogen atoms and a weighing scheme of the
2
ꢀꢃ
Hydrogen atom positions were calculated for idealized
ꢁ
ꢂ
ꢄ
ꢁ
4
10
2
2
0
2
0
form W ¼ 1 r F þ ð0:ap) þ bp , where P ¼ F þ
vacuo.
ꢀ
2
2
F Þ 3.
c
Microbiological test methods
positions and treated with the riding model option of
SHELXL, except of those H-atoms that were bound to
nitrogen atoms. Their positions were derived from the final
Fourier maps and refined. Hydrogen atoms linked to carbon
The in vitro antimicrobial activities were tested using the
disk diffusion method [16]. The viable bacterial cells were
swabbed onto Muller Hinton Agar (MHA) plates and
1
23

Transition Met Chem (2011) 36:417–424
419
Table 1 Crystal data and
structure refinement parameters
of [Co(HL) (MeOH) ](NO
Parameters
[Co(HL)
2 2 3 2
(MeOH) ](NO )
2
2
3 2
)
CCDC no.
802092
Empirical formula
Formula weight (M)
Temperature (T) (K)
22 8 10
C H34CoN O
629.5
293
˚
Wavelength (Mo Ka) (A)
Crystal system
0.71073
Orthorhombic
Space group
Pbca
˚
Unit cell dimensions
a (A) = 15.7192(5)
˚
b (A) = 11.1112(4)
˚
c (A) = 16.2768(7)
a (°) = 90
b (°) = 90
c (°) = 90
2,842.89(18)
8
3
Volume V (A )
˚
Z
3
D
calc (q) (Mg/m )
1.471
-1
)
Absorption coefficient, l (mm
0.671
F
(000)
1,316
3
Crystal size (mm )
Color, nature
0.25 9 0.20 9 0.20
Brown, block
2.50–28.50
3,596 (0.0300)
-19 B h B 21
h Range for data collection (°)
Independent reflections (Rint
)
Limiting indices
-
-
13 B k B 14
21 B l B 19
Reflections collected
18,035
Observed reflections [(I [ 2r (I)]
Completeness to h
2,534
2,850° (99.9%)
Semiempirical from equivalents
Absorption correction
Maximum and minimum transmission
Refinement method
0.8776 and 0.7903
2
Full-matrix least squares on F
Data/restraints/parameters
2
Goodness-of-fit on F
3596/2/199
1.024
Final R indices [(I [ 2r (I)]
R
1
= 0.0339
= 0.0833
= 0.0566
= 0.0935
0.285 and -0.334
wR
2
R indices (all data)
R
1
wR
2
R1 ¼ RjjF0j ꢁ jFcjj=RjF0
Largest difference peak and hole (e A˚ -3
h
.
i
)
ꢁ
ꢂ
ꢁ
ꢂ
1=2
2
2
2
2
2
wR2 ¼ Rw F ꢁ Fc
Rw F
0
0
atoms were located via geometrical constraints. All esds,
except the esd in the dihedral angle between two least
square planes, were estimated using the full covariance
Results and discussion
The colors, partial elemental analyses, molar conductivities,
and magnetic susceptibility values of the complexes are
presented in Table 2. In all the complexes, the cobalt is in the
2
matrix. Refinement of F was done against all reflections.
A total of 18,035 (3,596 unique, Rint = 0.0300) reflections
form (-19 B h B 21, -13 B k B 14, -21 B l B 19)
were collected. Absorption corrections were employed
using multi-scan [19] (Tmax = 0.8776 and Tmin = 0.7903).
The graphical tool used was DIAMOND Version 3.1f [20].
2
?
?2 oxidation state. In [Co(HL) ](OAc) , the Co nucleus
2
2
appears to be four coordinated with two neutral ligand
moieties, whereas the acetate ion remained outside the
coordination sphere. This stoichiometry was in accordance
123

4
20
Transition Met Chem (2011) 36:417–424
Table 2 Analytical data of ligand (HL) and its Co(II) complexes
a
m
b
Compound
Color
Composition % found (calc)
K
m
l (B.M.)
Carbon
Hydrogen
Nitrogen
HL
Colorless
Brown
62.8 (62.8)
51.5 (51.5)
46.8 (46.9)
41.6 (41.9)
44.6 (44.7)
6.6 (6.9)
5.8 (5.8)
5.2 (5.1)
5.3 (5.5)
4.9 (4.9)
21.9 (21.9)
14.9 (15.0)
16.4 (16.4)
17.8 (17.8)
15.6 (15.6)
[
[
[
[
a
b
Co(HL)
Co(HL)
Co(HL)
2
2
2
](OAc)
Cl
(MeOH)
(SO
2
87
15
93
13
4.4
5.2
5.1
4.9
2
]
Light brown
Brown
2 3 2
](NO )
Co
2
(HL)
4
4
)
2
]
Brown
-3
Molar conductivity, 10 M methanol at 298 K
Magnetic susceptibility per metal atom
with the elemental analyses results. For [Co(HL) (MeOH) ]-
2
2
(
NO ) , a possible stoichiometry with two neutral ligand
3 2
units coordinated to the metal center along with a nitrate
group in the outer sphere was suggested by the elemental
analysis, and the structure was confirmed by single-crystal
X-ray crystallography. The coordination geometries of the
other complexes were deduced from their spectra. Conduc-
tivity measurements of the complexes [Co(HL) ](OAc)
2
2
-
1
2
-1
(
(
K , 87 Ohm cm mol ) and [Co(HL) (MeOH) ]-
m
2
2
-
1
2
-1
NO ) (K , 93 Ohm cm mol ) show that they are
3
2
m
electrolytes, whereas complexes [Co(HL) Cl ] and [Co -
2
2
2
(
HL) (SO ) ] are found to be non-electrolytes in methanol
4 4 2
solution.
Crystal structure of [Co(HL) (MeOH) ](NO )
3 2
2
2
A perspective view of the compound showing the crystal-
lographic numbering scheme is shown in Fig. 1 and
selected bond lengths and angles are collected in Table 3.
The molecule is centrosymmetric and one half of the
molecule is related to the other half by a twofold axis
passing through the cobalt atom. The complex crystallizes
in orthorhombic space group Pbca. The unit cell contains
eight molecules. The six-coordinate octahedral Co(II)
complex contains two neutral bidentate ligands (HL), two
methanol ligands, and two non-coordinating nitrate anions.
The semicarbazone ligands constitute the equatorial plane
of the octahedron with the metal at the center. The two
methanol molecules occupy the remaining two axial posi-
tions and hence, they are mutually trans. The Co atom is
coordinated by azomethine nitrogen N(1) and carbonyl
oxygen O(2) from the semicarbazone, and oxygen O(1)
from methanol. The perspective view of the complex
shows that the semicarbazone functions as an ON donor
ligand, forming two five-membered chelate rings. The
compound is centrosymmetric with closely associated
crystallographically equivalent molecules, with the Co
atom situated on an inversion center in which the bond
Fig. 1 Molecular structure of [Co(HL)
the atom numbering scheme
2
2 3 2
(MeOH) ](NO ) showing
0
Co(1)–N(l ) are 180°. The semicarbazone fragments {[O(2)
and N(l )], [O(2) and N(1)]} are coordinated to the Co(II)
0
0
center with angles of O(2)–Co(1)–N(1 ) [103.14(5)°] and
O(2)–Co(1)–N(1) [76.86(5)°], respectively, which are quite
far from a perfect octahedron indicating considerable dis-
tortion in the geometry. The terminal aromatic ring is
twisted out of the plane as seen in the value of the C(1)–
N(3)–C(5)–C(10) torsion angle of -50.7(2)°. The non-
coordinating nitrate anions are quite far from a perfect
trigonal planar structure, as indicated from their bond
angles.
Bond distances within the semicarbazone ligand, par-
˚
ticularly the C(1)–O(2) distance [1.2427(18) A], which is
˚
longer than the standard C=O bond length (1.21 A) [21],
together with the presence of the N–H proton, clearly
indicate that the semicarbazone ligand is bound to cobalt in
0
0
angles of O(2)–Co(1)–O(2 ), O(1)–Co(1)–O(l ) and N(1)–
1
23

Transition Met Chem (2011) 36:417–424
421
˚
angles of HL [15]. The O(2) and N(1) atoms are in
E conformation with respect to the C(1)–N(2) bond in the
free ligand, whereas in the complex they are in the Z form.
The torsion angles N(2)–N(1)–C(2)–C(4), -179.78 (17)°
and N(1)–N(2)–C(1)–O(2) -4.0 (2)° indicate the trans
configuration of the C(2)–N(1) bond and the cis configura-
tion of the C(1)–N(2) bond.
Table 3 Selected bond lengths (A) and bond angles (°) of [Co(HL)
2
-
(
2 3 2
MeOH) ](NO )
Bond lengths
Bond angles
C(1)–O(2)
C(1)–N(3)
C(1)–N(2)
C(2)–N(1)
C(5)–N(3)
N(1)–N(2)
N(1)–Co(1)
N(2)–H(2)
N(3)–H(3)
N(4)–O(5)
N(4)–O(3)
N(4)–O(4)
C(11)–O(1)
O(1)–Co(1)
O(2)–Co(1)
1.243(18)
1.343(2)
1.348(2)
1.279(2)
1.414(2)
1.390(18)
2.176(13)
0.838(9)
0.831(9)
1.208(2)
1.218(2)
1.239(19)
1.418(2)
2.111(12)
2.044(10)
C(11)–O(1)–Co(1)
C(1)–O(2)–Co(1)
124.48(12)
116.14(10)
179.99(1)
91.93(5)
0
O(2)–Co(1)–O(2 )
O(2)–Co(1)–O(1)
0
The packing diagram of the compound viewed along a
axis is shown in the supplementary data. The pꢀꢀꢀp inter-
actions are weak as they correspond to a distance greater
O(2 )–Co(1)–O(1)
88.07(5)
˚
than 3.8 A. However, there are appreciable C–Hꢀꢀꢀp inter-
0
O(1)–Co(1)–O(1 )
0
180
actions, of which the interaction at a HꢀꢀꢀCg distance of
O(2)–Co(1)–N(1 )
103.14(5)
˚
2
.549 A is the most prominent. The C–Hꢀꢀꢀp interactions of
the O–H and methyl hydrogens of the methanol with the
rings of the neighboring molecules contribute to the unit
cell packing. The C(3) and C(4) methyl hydrogen of the
semicarbazone are also involved in C–Hꢀꢀꢀp interactions
with the N(4)-phenyl ring Cg(3). The molecules are con-
nected by various intermolecular hydrogen bonds involving
the nitrate oxygen atoms (Fig. 2). Oxygen O(1) of the
methanol is intermolecularly hydrogen-bonded to O(4) of
the nitrate anion, while N(2) and N(3) of the semicarba-
zone moiety are intermolecularly hydrogen-bonded to O(3)
of the nitrate anion to give a six-membered ring comprising
of atoms N(3), H(3), O(3), H(2), N(2), and C(1). It is
interesting to note that H(3) forms two intermolecular
hydrogen bonds with O(3) and O(5) of the neighboring
nitrate anion [N(3)–H(3)ꢀꢀꢀO(3); N(3)–H(3)ꢀꢀꢀO(5)] [sym-
metry code: 1 - x, -1 - y, 1 - z with a H(3)ꢀꢀꢀO(3) and
O(2)–Co(1)–N(1)
O(1)–Co(1)–N(1)
76.86(5)
92.85(5)
87.15(5)
180
0
O(1 )–Co(1)–N(1)
0
N(1 )–Co(1)–N(1)
the keto form (Fig. 1). The N(2)–C(1) bond length of
˚
.348(2) A is considerably shorter than the standard N–C
˚
single bond length [1.47 A]. The azomethine bond, C(2)–
˚
N(1) distance 1.279(2) A, is close to an ideal C=N double
˚
bond [1.28 A], confirming the azomethine bond formation
˚
22]. The N(1)–N(2) [1.3900(18) A] and N(2)–C(1)
˚
1.348(2) A] bond distances are intermediate between ideal
˚
values of corresponding single [N–N, 1.45 A, C–N, 1.47 A]
˚
and double [N=N, 1.25 A, C=N, 1.28 A] bonds, giving
1
˚
[
[
H(3)ꢀꢀꢀO(5) distance 2.364(13) and 2.420(13) A] forming a
four-membered ring comprising of atoms H(3), O(5), N(4),
and O(3). A weak intermolecular hydrogen bond [C(3)–
H(3B)ꢀꢀꢀO(4)] is observed with an angle of 159°, and there
is also a weak intramolecular hydrogen bond between
C(14)–H(4C) and O(2) with an angle of 150°. Hydrogen
bonds and other interactions are summarized in Table 4.
˚
˚
evidence for an extended p-delocalization along the semi-
carbazone chain [23]. The Co–O(methanol), Co–N(azomethine)
,
and Co–O(carbonyl) bond distances are 2.1114(2), 2.1764(13),
˚
and 2.0439(10) A, respectively. The variations in Co–O
bond distances indicate the differences in the strength of the
bonds formed by each of the coordinating O atoms and can
be attributed to the difference in the p-back bonding capa-
bilities of the oxygen of methanol and the semicarbazone
moieties. The Co(1)–N(1) bond distance is greater than
the Co–N bond distances reported for other octahedral cobalt
complexes [14], which suggests a lack of significant
p-bonding in this case. It is evident that there is a significant
elongation of the C(1)–O(2) bond distance with a concom-
itant reduction in N(2)–C(1) (which is a part of the chelate
ring) upon coordination of HL. The changes in bond dis-
tances are accompanied by changes in the bond angles in HL
when the five-membered chelate rings are formed in the
complex. It is notable that the N(1)–N(2)–C(1) bond angle
contracts by about 3.18° and N(1)C(2)C(4) angle widens by
about 1.82° when compared to the previously reported bond
Spectroscopic and magnetic studies
The tentative assignments of the main IR bands of the
complexes are presented in Table 5. The characteristic
2
m( NH) and m(C=O) stretches are observed at 3,196 and
1,682 cm for free HL, indicative of the ketonic nature of
-
1
the semicarbazone in the solid state [24]. For all the
2
complexes, the presence of a band assigned to m( NH)
indicates the semicarbazone is coordinated in the neutral
form, and the downward shift in the m(C=O) stretch for the
complexes supports the coordination of the keto form of
the ligand, as confirmed by the X-ray crystal structure of
the complex [Co(HL) (MeOH) ](NO ) . The strong bands
2
2
3 2
-
1
in the region 3,206–3,340 cm
in the spectra of the
4
complexes have been assigned to m( NH) vibrations [25].
123

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Transition Met Chem (2011) 36:417–424
2 2
Table 4 Hydrogen bonds and short contacts in [Co(HL) (MeOH) ]-
(
NO
3 2
)
(
1) H bonding
˚
D–HꢀꢀꢀA
O(1)–H(1A)ꢀꢀꢀO(4) 0.82
D–H (A) HꢀꢀꢀA ( A˚ ) DꢀꢀꢀA ( A˚ ) D–HꢀꢀꢀA (°)
a
1.87 2.690(17) 176.4
b
N(2)–H(2)ꢀꢀꢀO(3)
N(3)–H(3)ꢀꢀꢀO(3)
N(3)–H(3)ꢀꢀꢀO(5)
0.84(9)
2.07(12) 2.859(2) 157(2)
b
b
0.83(9)
2.36(13) 3.101(3) 148.1(17)
0.83(14) 2.42(13) 3.212(3) 159.8(16)
c
C(3)–H(3B)ꢀꢀꢀO(4) 0.96
2.60
2.27
3.510(3) 159
3.136(2) 150
a
C(4)–H(4C)ꢀꢀꢀO(2) 0.96
(
2) C–Hꢀꢀꢀp interactions
X–H(I)ꢀꢀꢀCg(J)
HꢀꢀꢀCg ( A˚ )
X–HꢀꢀꢀCg (°)
XꢀꢀꢀCg ( A˚ )
a
O(1)–H(1A)ꢀꢀꢀCg(1)
O(1)–H(1A)ꢀꢀꢀCg(2)
2.539
2.539
2.871
3.029
2.999
2.999
88.42
88.42
2.647(13)
2.647(13)
3.614(2)
3.973(2)
3.408(2)
3.408(2)
Fig. 2
Co(HL)
Intermolecular hydrogen bonding interactions of
(MeOH) ](NO
b
[
2
2
3 2
)
c
C(3)–H(3A)ꢀꢀꢀCg(3)
C(4)–H(4A)ꢀꢀꢀCg(3)
135.00
167.88
107.08
107.08
d
b
a
C(11)–H(11C)ꢀꢀꢀCg(1)
C(11)–H(11C)ꢀꢀꢀCg(2)
-
1
Free HL displays a strong sharp band at 1,591 cm
ascribed to m(C=N) of the azomethine group. For the
complexes, m(C=N) shows an upward shift to the region of
D Donor, A acceptor, Cg centroid
-
,598–1,610 cm , confirming the coordination of azo-
1
1
Notes
a
(1) Equivalent position code: 1 - x, -y, 1 - z, 1 - x, -1 - y,
1
b
methine nitrogen [26]. This is further supported by the
c
- z, x, -1/2 - y, -1/2 ? z
-
1
presence of a band at 415–421 cm which corresponds to
the m(Co–N) stretch [27]. The spectra of the complexes
exhibit a systematic shift in the positions of the m(N–N)
a
2) Equivalent position code: 1 - x, -y, 1 – z, x, y, z, x, -1/2
-
b
c
(
d
y, -1/2 ? z, -1/2 ? x, -1/2 - y, 1 - z
0
0
-
1
Cg(1) = Co(1), O(2), C(1), N(2), N(1); Cg(2) = Co(1), O(2 ), C(1 ),
0
bands in the 1,140–1,183 cm region which gives further
evidence of coordination of the ligand through the imine
nitrogen [28]. The increase in the m(N–N) value in the
spectra of the complexes is explained by the increase in
0
N(2 ), N(1 ); Cg(3) = C(5), C(6), C(7), C(8), C(9), C(10)
-
Table 5 Infrared spectroscopic assignments (cm ) of the ligand
1
(
HL) and its Co(II) complexes
-
1
double bond character. A band at ca. 1,501 cm
is
2
4
Compound
m NH m NH mC=O mC=N mN–N mM–N
assigned the interaction between N–H bending and C=N
stretching vibration of the C–N–H group of the amide
HL
3,196 3,375 1,682 1,591 1,129
-
1
function [29]. A weak band at ca. 1,257 cm also results
from the N–H bending and C–N stretching interactions. For
the complex [Co(HL) (MeOH) ](NO ) , a weak band at
[Co(HL) ](OAc)
2
3,162 3,340 1,657 1,598 1,140 420
3,165 3,254 1,651 1,604 1,149 415
3,162 3,294 1,655 1,600 1,144 418
3,156 3,206 1,658 1,610 1,183 421
2
[Co(HL)
[Co(HL)
2
2
Cl
(MeOH)
(SO
2
]
2 3 2
](NO )
2
2
3 2
-
1
5
62 cm indicates the presence of a Co–O bond resulting
[Co (HL)
2
4
4
)
2
]
from the coordination of methanolic oxygen.
The spectrum of the acetato complex [Co(HL) ](OAc)
2
2
-
1
displays strong bands at 1,620 cm
1
[m (COO)] and
,314 cm [m (COO)], indicating the ionic nature of the
a
-
The electronic spectrum of free HL shows a maximum at
1
s
243 nm assigned to p ? p* transitions of the imine func-
acetate in this compound [30]. For [Co(HL) (MeOH) ]-
2
2
tion, while a band at 294 nm is assigned to n ? p* transi-
tions of the azomethine and carbonyl groups of the
semicarbazone moiety [33]. These are shifted to higher
energies upon complexation. Bands at ca. 385 nm in the
(
NO ) , the presence of bands at 823 (m ), 1,384 (m ), and
3
2
2
3
-
1
7
00 cm (m ) clearly points to the uncoordinated nature of
4
the nitrate group [31]. For [Co (HL) (SO ) ], the m and m
vibrations, observed as weak bands at 910 and 470 cm
2
4
4 2
1
2
-
1
,
II
spectra of the complexes are assigned to O ? Co and
suggest the bridged bidentate nature of the sulfate anion in
this complex [32]. The symmetry of the sulfate is reduced
to C2V when it functions as a bidentate ligand in the
II
N ? Co LMCT transitions. The Co(II) complexes
[
Co(HL) Cl ], [Co(HL) (MeOH) ](NO ) and [Co (HL) -
2 2 2 2 3 2 2 4
(
SO ) ] all showed d–d bands in the 670–531 nm region.
4 2
complex. The m vibrations are observed at 1,054, 1,125,
3
1
4
4
-
These are assigned to the transitions ( T (F) ? A (F)]
(
and 1,250 cm , while the m vibrations are observed near
1g
2g
4
4
4
-
1
m ) and ( T (F) ? T (P)] (m ), consistent with their
6
10 and 507 cm
.
2
1g
1g
3
1
23

Transition Met Chem (2011) 36:417–424
423
-1
-1
Table 6 UV–Vis spectra kmax, nm (e, M cm ) in methanol solution and assignments
Compound
p ? p*
n ? p*
LMCT
d–d
HL
243 (27,250)
215 (29,500)
210 (30,000)
233 (29,000)
212 (28,750)
294 (23,750)
254 (26,500)
265 (29,000)
278 (27,500)
265 (21,000)
–
–
[
[
[
[
Co(HL)
Co(HL)
Co(HL)
2
2
2
](OAc)
Cl
(MeOH)
(SO
2
371 (22,000)
385 (22,000)
356 (23,000)
381 (17,500)
720 (11,500), 708 (19,000)
639 (18,900), 551 (19,750)
644 (17,500), 550 (20,000)
670 (14,000), 531 (15,500)
2
]
2 3 2
](NO )
Co
2
(HL)
4
4
)
2
]
Table 7 Antibacterial and antifungal screening data
Compound
Zone of inhibition (mm)
Bacteria
Fungi
E. coli
S. typhi
P. vulgaris
E. aerogenes
B. megaterium
A. niger
C. albicans
[
Co(HL) Cl
2
2
]
10.5
17
14
–
–
14
10
–
9
–
–
7
7.5
–
15.5
–
8
7
[
Co(HL) (MeOH)
2
2 3 2
](NO )
Penicillin
Gresiofulvin
15
–
12
–
20
–
–
–
14
10
Concentration 20 lg/disk. The free ligand HL showed no activity against any of the test organisms
octahedral configuration [34]. Electronic spectral data for
HL and its complexes are summarized in Table 6.
antifungal screening data of two complexes are presented
in Table 7. Chelation often increases the antibacterial
activity of a given ligand, due to lipophilic character of the
complexes that favor permeation through the bacterial
membranes [37]. In the present case, the results reveal that
the complexes show some activity, whereas the free ligand
is inactive against the tested pathogenic bacteria and fungi.
The observed magnetic moments of the Co(II) com-
plexes are given in Table 2. Both tetrahedral and octahedral
Co(II) complexes posses three unpaired electrons, but may
be distinguished by the magnitude of the deviation of leff
from the spin-only value. The magnetic moment of tetra-
hedral Co(II) complexes with an orbitally non-degenerate
ground term is increased above the spin-only value via
contribution from higher orbitally degenerate terms and
occurs in the range 4.2–4.7 B.M. [35]. Octahedral Co(II)
Supplementary material
4
All crystallographic data for this paper are deposited with
the Cambridge Crystallographic Data Centre (CCDC—
complexes maintain a large contribution due to the T
1
g
ground term and exhibit leff in the range 4.8–5.6 B.M.
802092). The data can be obtained free of charge at
[
36]. Room temperature magnetic susceptibility studies on
www.ccdc.cam.ac.uk/conts/retrieving.html [or from Cam-
bridge Crystallographic Data Centre (CCDC), 12 Union
Road, Cambridge CB2 1EZ, UK; fax: ?44 (0) 1223-
[
Co(HL) ](OAc) gave a l value of 4.4 B.M., as expected
2
2
eff
of a four-coordinate tetrahedral Co(II) complex. The leff
values for [Co(HL) Cl ], [Co(HL) (MeOH) ](NO ) , and
2
2
2
2
3 2
336033; e-mail: deposit@ccdc.cam.ac.uk].
[
Co (HL) (SO ) ] were 5.2, 5.14, and 4.9 B.M., showing
2 4 4 2
that these complexes have an octahedral configuration.
Acknowledgments V. L. Siji is thankful to the Council of Scientific
and Industrial Research, New Delhi, India, for financial support in the
form of Junior Research Fellowship. The authors are thankful to Dr.
M. R. P. Kurup, Department of Applied Chemistry, Cochin University
of Science and Technology, for using the software DIAMOND ver-
sion 3.1f. We are grateful to Dr. Babu Varghese, SAIF, IIT Chennai,
India, for providing single-crystal XRD data. The authors are thankful
to the SAIF, Cochin University of Science and Technology, Kochi,
India, for elemental analyses, IR and electronic spectral data and IIT
Roorkee, India, for magnetic susceptibility measurement.
Antimicrobial studies
Free ligand and its two complexes, [Co(HL) Cl ] and
2
2
[
Co(HL) (MeOH) ](NO ) , were screened against five
2 2 3 2
bacterial cultures, namely Escherichia coli MTCC 585,
Salmonella typhi MTCC 734, Proteus vulgaris MTCC
1
771, Enterobacter aerogenes MTCC 2990, and Bacillus
megaterium MTCC 2248 and two fungal cultures, namely
Aspergillus niger MTCC 281 and Candida albicans MTCC
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