Microwave-assisted synthesis, crystal structures, and in vitro antibacterial studies of cobalt(III) and vanadium(V) complexes derived from N,N′-1,2-propylene-bis(3-methylsalicylideneimine)

Article abstract of DOI:10.1080/15533174.2012.760599

The syntheses of a mononuclear cobalt(III) complex [CoL(NCS) (OH ₂)] and a mononuclear vanadium(V) complex [VOL(NCS)] (H₂L = N,N′-1,2-propylene-bis(3-methylsalicylideneimine)), by a one-step sequence from the reaction of H₂L, ammonium thiocyanate and metal salts under microwave irradiation are described. The newly prepared complexes were characterized by a combination of elemental analyses and IR spectra. Their structures were determined by single-crystal X-ray crystallography. Both complexes with octahedral metal centers have similar structures. The crystal of the cobalt complex is stabilized by hydrogen bonds, while there are no such interactions in the crystal of the vanadium complex. In order to evaluate the biological activity of H₂L and to evaluate the role of cobalt and vanadium ions on biological activity, the free ligand and the complexes have been studied in vitro antibacterial against Staphylococcus aureus, Bacillus subtilis, Escherichia coli, and Pseudomonas aeruginosa. Copyright Taylor & Francis Group, LLC.

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Microwave-Assisted Synthesis, Crystal Structures,
and In Vitro Antibacterial Studies of Cobalt(III) and
Vanadium(V) Complexes Derived From N,N’-1,2-
propylene-bis(3-methylsalicylideneimine)
Wei-Guang Zhang a & Ji-Hong Liang b
a College of Chemistry and Chemical Engineering , Qiqihar University , Qiqihar , P. R. China
b Library, Qiqihar University , Qiqihar , P. R. China
Accepted author version posted online: 24 May 2013.Published online: 25 Aug 2013.
To cite this article: Wei-Guang Zhang & Ji-Hong Liang (2013) Microwave-Assisted Synthesis, Crystal Structures, and
In Vitro Antibacterial Studies of Cobalt(III) and Vanadium(V) Complexes Derived From N,N’-1,2-propylene-bis(3-
methylsalicylideneimine), Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 43:10, 1355-1360,
DOI: 10.1080/15533174.2012.760599
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 43:1355–1360, 2013
Copyright ꢀ Taylor & Francis Group, LLC
C
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2012.760599
Microwave-Assisted Synthesis, Crystal Structures,
and In Vitro Antibacterial Studies of Cobalt(III)
and Vanadium(V) Complexes Derived From
N,N’-1,2-propylene-bis(3-methylsalicylideneimine)
Wei-Guang Zhang1 and Ji-Hong Liang2
1College of Chemistry and Chemical Engineering, Qiqihar University, Qiqihar, P. R. China
2Library, Qiqihar University, Qiqihar, P. R. China
from salicylaldehyde and its analogues are reported to have in-
teresting biological properties.[6–8] When bioorganic molecules
or drugs are bound to metal ions, there is drastic change in their
biomimetic properties, therapeutic effects, and pharmacological
properties. Essential metal ions such as cobalt and vanadium and
their complexes are found to be antitumor active, catalytic active,
antimicrobial, and cytotoxic.[9–12] In this study, the synthesis,
characterization, and antibacterial properties of a mononuclear
cobalt(III) complex [CoL(NCS)(OH2)] and a mononuclear
The syntheses of a mononuclear cobalt(III) complex [CoL(NCS)
(OH2)] and a mononuclear vanadium(V) complex [VOL(NCS)]
(H2L = N,Nꢁ-1,2-propylene-bis(3-methylsalicylideneimine)), by a
one-step sequence from the reaction of H2L, ammonium thio-
cyanate and metal salts under microwave irradiation are described.
The newly prepared complexes were characterized by a combina-
tion of elemental analyses and IR spectra. Their structures were de-
termined by single-crystal X-ray crystallography. Both complexes
with octahedral metal centers have similar structures. The crystal
of the cobalt complex is stabilized by hydrogen bonds, while there
are no such interactions in the crystal of the vanadium complex.
In order to evaluate the biological activity of H2L and to evaluate
the role of cobalt and vanadium ions on biological activity, the free
ligand and the complexes have been studied in vitro antibacterial
against Staphylococcus aureus, Bacillus subtilis, Escherichia coli,
and Pseudomonas aeruginosa.
vanadium(V) complex [VOL(NCS)] (H2L
propylene-bis(3-methylsalicylideneimine)) are described.
=
N,Nꢀ-1,2-
EXPERIMENTAL
Chemicals and Physical Measurements
All the starting materials and solvents used in the present
investigation were of analytical grade and used without fur-
ther purification. WX-4000 microwave digestion system (APL
Instrument Co., Ltd, Chengdu, P. R. China) was used in mi-
crowave synthesis. Elemental analyses were performed on a
Perkin-Elmer 2400 Elemental Analyzer (Qiqihar University,
P. R. China). IR spectra were recorded as KBr pellets on a
Bio-Rad FTS 135 spectrophotometer (Qiqihar University, P. R.
China) in the range of 4000–400 cm−1. Conductivities of 10−3
M solutions in acetonitrile were measured on a DDS-11A con-
ductivity meter (Qiqihar University, P. R. China).
Keywords antibacterial activity, cobalt complex, crystal structure,
microwave-assisted synthesis, vanadium complex
INTRODUCTION
Microwave-assisted synthesis as a new and extremely attrac-
tive method is emerging in the area of liquid-phase inorganic
synthesis.[1–3] Microwave irradiation can accelerate many chem-
ical reactions. This method of synthesis has also advantages
in providing a clean, cheap, and easy handling heating way,
which achieves not only higher yields with less reaction time,
but more complete products.[4,5] Metal-organic coordination
compounds have received great interest in the fields of crystal
engineering and functional materials, due to their intriguing
structural features as well as mysterious properties. Schiff bases
have been known for a long time. Most Schiff bases derived
Preparation of H2L
To a hot ethanolic (20 mL) solution of 3-methylsali
cylaldehyde (1.36 g, 1.00 mmol), 1,2-diaminopropane (1.48 g,
2.00 mmol) in ethanol (20 mL) was added dropwise with con-
stant stirring and refluxed for 3 h after cooling the mixture
to room temperature. The precipitated product thus formed
was filtered off, washed with ice-cooled ethanol, and recrys-
tallized from the same solvent. The yellow crystalline product
of the compound was dried in air. Yield, 78%. Anal. Calcd. for
C19H22N2O2: C, 73.52; H, 7.14; N, 9.02; Found: C, 73.40; H,
7.22; N, 9.11%. Selected IR: 3269 (w, –OH), 1630 (s, C N),
1281 (s, C–N), 1250 (w, C–OH).
Received 10 December 2012; accepted 16 December 2012.
Financial assistance from the Qiqihar University is gratefully ac-
knowledged.
Address correspondence to Wei-Guang Zhang, College of Chem-
istry and Chemical Engineering, Qiqihar University, Qiqihar 161006,
P. R. China. E-mail: zhangweiguang1230@163.com
1355

1356
W.-G. ZHANG AND J.-H. LIANG
63% (based on V). Anal. Calcd. for C20H20N3O3SV: C, 55.43;
Preparation of the Complexes
H, 4.65; N, 9.70; Found: C, 55.30; H, 4.57; N, 9.60%. Selected
IR: 2072 (s, NCS), 1617 (s, C N), 1266 (m, C–N), 1245 (w,
[CoL(NCS)(OH2)]
H2L (56.5 mg, 0.2 mmol), ammonium thiocyanate (15.2 mg,
0.2 mmol), Co(CH3COO)2·4H2O (50.0 mg, 0.2 mmol), and
15 mL methanol were placed in a 30-mL Teflon-lined autoclave,
which was then inserted into the cavity of a microwave reac-
tor. The reaction mixture was maintained at 350 K and 200 W
for 10 min. Then natural cooling was followed for about 1 h.
The resulting solution was then filtered and allowed to evapo-
rate slowly at room temperature. Red block-like single crystals
appeared in five days. The crystals were filtered off, washed
with water and dried in air. Yield, 45% (based on Co). Anal.
Calcd. for C20H22CoN3O3S: C, 54.17; H, 5.00; N, 9.48; Found:
C, 54.28; H, 5.12; N, 9.37%. Selected IR: 3432 (m, –OH),
2067 (s, NCS), 1618 and 1616 (s, C N), 1267 (m, C–N), 1236
C–OH). Molar conductance value: 8 ꢀ−1 cm2 mol−1
.
Structure Determination
Single-crystals X-ray diffraction analyses of the complexes
were carried out on a Bruker Smart Apex CCD diffractometer
(Heilongjiang University, P. R. China) equipped with a graphite
monochromated Mo Kα radiation (λ = 0.71073 Å) at 298(2) K.
Raw frame data were integrated with the SAINT program.[13]
The structures were solved by direct methods with SHELXS-97
and refined by full-matrix least-squares on F2 using SHELXS-
97.[14] An empirical absorption correction was applied with
the program SADABS.[15] All non-hydrogen atoms were re-
fined anisotropically. The hydrogen atoms of the water ligand in
the cobalt complex were located from electronic density map.
Molecular graphics software used was ORTEP III.[16] The crys-
tal data for the complexes are listed in Table 1. Selected bond
lengths and angles for the complexes are listed in Table 2.
(w, C–OH). Molar conductance value: 13 ꢀ−1 cm2 mol−1
.
[VOL(NCS)]
The same synthetic process as described of the cobalt com-
plex was used except Co(CH3COO)2·4H2O was replaced by
VO(acac)2 (53.0 mg, 0.2 mmol). The brown block-like single
crystals of the complex appeared in a week by slow evaporation
of the solution, which were filtered off and dried in air. Yield,
TABLE 2
Selected bond lengths (Å) and bond angles (◦)
TABLE 1
[CoL(NCS)(OH2)]
Bond lengths
Crystallographic data collection and structure refinement
Co1–N1
Co1–O1
Co1–N3
1.885(4) Co1–N2
1.889(3) Co1–O2
1.915(4) Co1–O3
1.890(4)
1.897(3)
1.941(4)
Parameter
[CoL(NCS)(OH2)] [VOL(NCS)]
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C20H22CoN3O3S C20H20N3O3SV
443.4
Monoclinic
C2/c
433.4
Monoclinic
P21/c
Bond angles
N1–Co1–O1
O1–Co1–N2
O1–Co1–O2
N1–Co1–N3
N2–Co1–N3
N1–Co1–O3
N2–Co1–O3
N3–Co1–O3
[VOL(NCS)]
Bond lengths
V1–O1
94.57(18) N1–Co1–N2
177.53(15) N1–Co1–O2 178.18(18)
84.9(2)
19.2107(9)
14.0079(6)
16.7646(7)
90
9.921(2)
13.599(3)
15.452(3)
90
87.25(13) N2–Co1–O2
90.69(17) O1–Co1–N3
90.58(18) O2–Co1–N3
90.43(17) O1–Co1–O3
91.37(18) O2–Co1–O3
177.83(17)
93.28(16)
91.84(16)
89.17(15)
86.21(16)
89.77(15)
b (Å)
c (Å)
α (◦)
β (◦)
116.8310(10)
90
101.303(2)
90
γ (◦)
V (Å3)
4025.7(3)
8
2044.3(7)
4
Z
T (K)
298(2)
1.463
298(2)
1.408
1.800(5) V1–O2
2.079(7) V1–N2
1.569(4) V1–N3
1.822(5)
2.089(7)
2.229(7)
Dcalc (g cm−3
)
V1–N1
V1–O3
μ (mm−1
F(000)
)
0.982
0.613
1840
896
Bond angles
O3–V1–O1
O1–V1–O2
O1–V1–N1
O3–V1–N2
O2–V1–N2
O3–V1–N3
O2–V1–N3
N2–V1–N3
Total data
Rint
Unique data
18949
0.0371
3744
8150
0.0347
1664
99.6(2)
109.2(2)
87.4(3)
90.8(3)
83.6(3)
173.5(2)
84.6(2)
83.6(2)
O3–V1–O2
O3–V1–N1
O2–V1–N1
O1–V1–N2
N1–V1–N2
O1–V1–N3
N1–V1–N3
98.0(2)
96.3(2)
155.8(3)
162.0(3)
76.8(3)
85.1(2)
79.3(2)
Observed data [I > 2σ(I)]
Data/restraints/parameters
Final R indices [I > 2σ(I)]a
R indices (all data)a
2794
1379
3744/3/264
0.0613
0.1727
1.077
ꢀ
1664/0/256
0.0546
0.1288
1.076
Goodness-of-fit on F2
ꢀ
ꢀ
ꢀ
aR = (|Fo – Fc|)/ |Fo|, wR = { [w(|Fo – Fc|)2]/ [w|Fo|2]}
.
1/2

COBALT(III) AND VANADIUM(V)CCOMPLEXES
1357
FIG. 2. ORTEP diagram of [VOL(NCS)] with atom labeling scheme. Thermal
ellipsoids are at 30% probability level.
and weak band at 1601 cm−1 are attributable to ν(O–H) stretch-
ing and δ(O–H) bending frequency of the coordinated water
molecule of the cobalt complex. The infrared spectra of the
complexes display very strong absorption at about 2070 cm−1
due to the NCS ligands. A strong absorption at 1630 cm−1
in the spectrum of the Schiff base is assigned to the azome-
thine group, ν(C N). This band undergoes a negative shift of
12–14 cm−1 in the complexes, which can be attributed to do-
nation of the nitrogen atom lone pair of the azomethine group
to the metal ions.[19] This assignment is further supported by
FIG. 1. ORTEP diagram of [CoL(NCS)(OH2)] with atom labeling scheme.
Thermal ellipsoids are at 30% probability level.
In Vitro Antibacterial Assay
H2L and the complexes were screened in vitro for their an-
tibacterial property against two Gram-positive (Staphylococ-
cus aureus MTCC 96, Bacillus subtilis MTCC 121) and two
Gram-negative (Escherichia coli MTCC 1652, Pseudomonas
aeruginosa MTCC 741) bacterial strains by agar well diffu-
sion method.[17] DMSO was used as a negative control, and
ciprofloxacin was used as positive control.
Determination of Minimum Inhibitory Concentration
MIC of the compounds against bacterial strains was tested
through a modified agar well diffusion method.[18] A twofold
serial dilution of each compound was prepared by first recon-
stituting the compound in DMSO followed by dilution in ster-
ile distilled water to achieve a decreasing concentration range
256 μM. A 100 μL volume of each dilution was introduced into
wells in the agar plates already seeded with 100 μL of stan-
dardized inoculums (106 cfu mL−1) of the test microbial strain.
All test plates were incubated aerobically at 37◦C for 24 h and
observed for the inhibition zones.
RESULTS AND DISCUSSION
Infrared Spectroscopic Characterization
The infrared spectra of the Schiff base ligand and the two
complexes provide information about the metal-ligand bond-
ing. The infrared spectra of the complexes are fully consistent
with their crystal structures. The broad band centered at 3432
FIG. 3. Packing diagram of [CoL(NCS)(OH2)] from the b view, showing the
hydrogen bonds.

1358
W.-G. ZHANG AND J.-H. LIANG
TABLE 3
Hydrogen bonding parameters (Å, ◦) for [CoL(NCS)(OH2)]
d(D–H) d(H···A) d(D···A)
(Å) (Å) (Å)
Angle
D–H···A
(D–H···A) (◦)
O3–H3A···O2i 0.85(1) 2.04(4) 2.794(5)
147(7)
166(5)
O3–H3B···S1ii 0.85(1) 2.71(2) 3.546(4)
Symmetry transformations used to generate equivalent atoms:
(i) 1 – x, y, 1/2 – z; (ii) x; 2 – y; 1/2 + z.
the presence of medium bands in the 520–550 cm−1 region for
the complexes, which are absent in the spectrum of the free
Schiff base. These bands can be assigned to ν(M–N). The phe-
nolic ν(C–O) in the free Schiff base exhibits a medium band at
1250 cm−1. In the complexes, this band appears at 1236 cm−1
for [CoL(NCS)(OH2)] and 1245 cm−1 for [VOL(NCS)]. The
weak bands in the 430 – 490 cm−1 region for both complexes
can be assigned to ν(M–O), and provides further evidence for
coordination through the deprotonated phenolic oxygen atom.
Description of Crystal Structures of the Complexes
Single-crystal X-ray diffraction analysis of the complexes re-
vealed that they are thiocyanato-coordinated mononuclear com-
plexes. The labeled diagrams of the cobalt and the vanadium
complexes are presented in Figures 1 and 2, respectively.
The Co ion of the cobalt complex is in an octahedral coordi-
FIG. 4. Packing diagram of [CoL(NCS)(OH2)] from the c view, showing the
1D columns running along the c axis.
nation, with the equatorial plane defined by two imino nitrogen N1-C8-C1-C2-O1 and N2-C12-C13-C14-O2 are 0.390(2) and
and two phenolate oxygen atoms of the Schiff base ligand, and 0.303(2) Å, respectively.
with two axial positions occupied by one oxygen atom of a water
In the molecular packing structure of the cobalt complex,
ligand and one nitrogen atom of a thiocyanate ligand. Obvious the mononuclear molecules are strongly hydrogen bonded
shortening in the equatorial bond lengths than in the axial bond (Figure 3) to each other by the water ligands through O–H···O
lengths clearly demonstrates the geometry of the central Co ion and O–H···S hydrogen bonds (Table 3) to give a 1D column
as an octahedral with some Jahn–Teller elongation in the axial along the c axis (Figure 4).
zones. The Co ion is slightly lifted out of the plane defined
The V ion of the vanadium complex is also in an octahedral
by the four equatorial donor atoms by an amount of 0.019(2) coordination, with the equatorial plane defined by two imino
Å. All the bond lengths and bond angles found for this com- nitrogen and two phenolate oxygen atoms of the Schiff base
plex are comparable to those in similar cobalt(III) complexes ligand, and with two axial positions occupied by one oxo oxy-
with octahedral coordination.[20,21] Both six-membered chelate gen atom and one nitrogen atom of a thiocyanate ligand. In the
rings have Co ion somewhat displaced from the mean plane of equatorial plane, the V–O bonds are much shorter than the V–N
the ligand atoms. Deviations of Co atom from the mean planes bonds, and in the axial positions, the V1 O3 bond is much
TABLE 4
Diameter of growth of inhibition zone (mm)a
Staphylococcus
aureus
Bacillus
subtilis
Escherichia
coli
Pseudomonas
aeruginosa
Compounds
H2L
13.7
21.5
17.7
24.3
15.5
18.3
22.6
21.7
11.0
19.7
–
12.1
16.4
–
[CoL(NCS)(OH2)]
[VOL(NCS)]
Ciprofloxacin
26.2
23.5
aDash (–) represents no activity.

COBALT(III) AND VANADIUM(V)CCOMPLEXES
1359
TABLE 5
MIC values (μM)a
Staphylococcus
Bacillus
subtilis
Escherichia
coli
Pseudomonas
aeruginosa
Compounds
aureus
H2L
128
64
32
128
32
16
8
256
32
–
128
32
–
[CoL(NCS)(OH2)]
[VOL(NCS)]
Ciprofloxacin
16
16
16
aDash (–) represents not determined.
shorter than the V1–N3 bond, indicating the octahedral coordi- CONCLUSION
nation is much distorted. The V ion is lifted out of the plane
defined by the four equatorial donor atoms by an amount of
0.221(2) Å toward the oxo oxygen atom. All the bond lengths
and bond angles found for this complex are comparable to those
in a similar vanadium(V) complex with octahedral coordina-
tion.[22] Both six-membered chelate rings have V ion somewhat
displaced from the mean plane of the ligand atoms. Deviations
of V atom from the mean planes N1-C8-C1-C2-O1 and N2-C12-
C13-C14-O2 are 0.369(2) and 0.818(2) Å, respectively. There
are no obvious short contacts among the molecules.
Using microwave assisted heating, two new structural similar
thiocyanato-coordinated cobalt and vanadium complexes with
octahedral coordination were obtained. The bonding of ligand
to metal ion is confirmed by elemental analyses and IR spectral
study, and conductance measurements. Structures of the com-
plexes have been confirmed by single-crystal X-ray diffraction
analysis. We found that microwave-assisted synthesis provides
a clean, cheap, and convenient method of heating that can result
in the final products with less reaction time. The antibacte-
rial property of the ligand and the complexes were evaluated,
which suggested that, in general, the metal complexes show
significantly enhanced antibacterial activity against the bacteria
strains in comparison to the free ligand. The vanadium complex
was found to be most effective against Bacillus subtilis.
Antibacterial Results
The antibacterial results are listed in Tables 4 and 5. The stud-
ies suggested that the free Schiff base H2L is biologically active
and the two complexes showed obviously enhanced antibacte-
rial activities in comparison to the free ligand. Ciprofloxacin
produced significantly sized inhibition zones against the tested
bacteria, while DMSO, the negative control, produced no in-
hibitory effect against any of the tested organisms. H2L shows
zone of inhibition ranging 11.0–15.5 mm against the four bac-
teria. The cobalt complex has been observed to show increased
zone of inhibition against the four bacterial strains as compared
to H2L. The vanadium complex has been observed to show in-
creased zone of inhibition against the Gram-positive bacteria,
while no activity against the Gram-negative bacteria. On the ba-
sis of zone of inhibition produced against the tested bacteria, the
cobalt complex was found to be most effective against Bacillus
subtilis with zone of inhibition of 18.3 mm. The MIC results
suggested that the free ligand showed weak activities against
the bacterial strains, and the complexes showed moderate ac-
tivities against the bacterial strains except for Escherichia coli
and Pseudomonas Aeruginosa, as compared to ciprofloxacin.
MIC results also revealed that the cobalt complex is more effec-
tive against the antibacterial strains as compared to the Schiff
bases. While for the vanadium complex, it is only more effec-
tive against the Gram-positive bacteria, but showed no activity
against the Gram-negative bacteria. The results of this study are
in accordance with those reported previously.[23] The overtone’s
concept[24] and Tweedy’s chelation theory[25] might be used to
explain the enhanced in antibacterial activity of the metal com-
plexes.
SUPPLEMENTARY MATERIALS
The X-ray crystallographic data in the CIF format for the
structures of the complexes reported in this paper have been de-
posited with the Cambridge Crystallographic Data Center, and
the supplementary crystallographic data can be obtained free of
charge on request at www.ccdc.cam.ac.uk/conts/retrieving.html
[or from The Director, Cambridge Crystallographic Data Cen-
ter, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; fax:
+44(0)1223-336033; email: deposit@ccdc.cam.ac.uk], quoting
the CCDC numbers 913223 and 913224.
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