Mixed-ligand aroylhydrazone complexes of molybdenum: Synthesis, structure and biological activity

Article abstract of DOI:10.1016/j.poly.2012.03.007

The reaction of the benzoylhydrazone of 2-hydroxybenzaldehyde (H ₂L) with [MoO₂(acac)₂] proceeds smoothly in refluxing ethanol to afford an orange complex [MoO₂L(C ₂H₅OH)] (1). The substrat

Full text of DOI:10.1016/j.poly.2012.03.007

Polyhedron 38 (2012) 198–204
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Mixed-ligand aroylhydrazone complexes of molybdenum: Synthesis, structure
and biological activity
a
a
a
a
b
b
Sagarika Pasayat , Subhashree P. Dash , Saswati , Paresh Kumar Majhi , Yogesh P. Patil , M. Nethaji ,
c
c
a,
⇑
Hirak R. Dash , Surajit Das , Rupam Dinda
a
Department of Chemistry, National Institute of Technology, Rourkela 769008, Orissa, India
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
Department of Life Science, National Institute of Technology, Rourkela 769008, Orissa, India
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of the benzoylhydrazone of 2-hydroxybenzaldehyde (H L) with [MoO (acac) ] proceeds
2
2
2
Received 16 December 2011
Accepted 4 March 2012
Available online 12 March 2012
smoothly in refluxing ethanol to afford an orange complex [MoO
2 2
L(C H
5
OH)] (1). The substrate binding
capacity of 1 has been demonstrated by the formation and isolation of two mononuclear [MoO
2
L(Q)]
0
{where Q = imidazole (2a) and 1-methylimidazole (2b)} and one dinuclear [(MoO
2 2
L) (Q)] {Q = 4,4 -bipyr-
idine (3)} mixed-ligand oxomolybdenum complex. All the complexes have been characterized by elemen-
tal analysis, magnetic and spectroscopic (IR, UV–Vis and NMR) measurements. The molecular structures
of all the oxomolybdenum(VI) complexes (1, 2a, 2b and 3) have been determined by X-ray crystallogra-
phy. In each complex, the dianionic planar ligand is coordinated to the metal centre via one enolate oxy-
gen, one phenolate oxygen and an azomethine nitrogen atom. The complexes have been screened for
their antibacterial activity against Escherichia coli, Bacillus and Pseudomonas aeruginosa. The minimum
inhibitory concentration of these complexes and their antibacterial activity indicates that compounds
Keywords:
Mixed-ligand aroylhydrazone complexes
Oxomolybdenum(VI)
X-ray crystal structure
Antibacterial activity
2a and 2b are potential lead molecules for drug designing.
Ó 2012 Elsevier Ltd. All rights reserved.
1
. Introduction
The coordination chemistry of molybdenum has become a fas-
most important pharmacophoric cores of several anti-inflamma-
tory, antinociceptive, and antiplatelet drugs [32]. It may be rele-
vant to mention here that although the study of the antibacterial
activity of aroylhydrazones complexes of many transition metals
have been extensively studied [33–38], that of oxomolybde-
num(VI) appears to have remained practically unexplored [39–41].
As part of our studies on aroylhydrazone complexes [29,33,42],
this article reports the synthesis, X-ray structure and characteriza-
cinating area of research in recent times because of the presence
of molybdenum in metalloenzymes and its biochemical as well
as catalytic importance [1–20]. Molybdenum Schiff’s base com-
plexes have continued to play the role of one of the most important
models of molybdoenzymes [21–23] due to their preparative
accessibility. Schiff bases derived from salicylaldehyde and amino
alcohols have been widely used for the preparation of oxomolyb-
tion of a new mononuclear Schiff base complex, MoO
(1), some mixed-ligand mononuclear cis-dioxomolybdenum(VI)
complexes, [MoO L(Imz)] (2a) and [MoO L(1-MeImz)] (2b), and a
2 2 5
L(C H OH)]
2+
denum complexes containing the MoO
2
-core [24–26], whereas
2
2
Schiff bases obtained by the reactions of aromatic acidhydrazides
with o-hydroxycarbonyl compounds have rarely been used in
molybdenum chemistry. This type of ligand is of particular interest
diimine bridged dinuclear cis-dioxomolybdenum(VI) complex,
0
2
[(MoO L)
2
(l-4,4 -bipy)] (3), with aroylhydrazone as a primary li-
gand, with special reference to their antibacterial activity.
2
because the MoO L or MoOL type of complex possesses one or two
‘
‘open’’ coordination sites that can be utilized for substrate binding
2
. Experimental
2.1. Materials
2
MoO (acac) ] was prepared as described in the literature [28].
properties [23,27–29] and these species have frequently been con-
sidered as models for enzymatic reactions and catalytic sites. On
the other hand, hydrazones –NH–N@CRR (R and R = H, alkyl, aryl)
are versatile ligands due to their applications in the field of analyt-
ical [30] and medicinal chemistry [31]. Hydrazone moieties are the
0
0
[
2
Reagent grade solvents were dried and distilled prior to use.
All other chemicals were reagent grade, available commercially
and used as received. Commercially available TBAP (tetra butyl
ammonium perchlorate) was properly dried and used as a
⇑
Corresponding author. Tel.: +91 661 246 2657; fax: +91 661 246 2022.
E-mail address: rupamdinda@nitrkl.ac.in (R. Dinda).
0
277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.03.007

S. Pasayat et al. / Polyhedron 38 (2012) 198–204
199
supporting electrolyte for recording cyclic voltammograms of the
complexes.
the solution, the color changed to dark orange. The solution mix-
ture was then refluxed for 3 h. After leaving the solution for 2 days
at room temperature, fine dark orange crystals were isolated and a
suitable single crystal was selected for X-ray analysis. Yield 0.31 g
2.2. Physical measurements
(
75%). Anal. Calc. for C16
Found: C, 46.55; H, 3.83; N, 7.06%. H NMR (DMSO–d
d: 9.01 (s, 1H, –N@C –H); 8.02–8.00 (m, 2H, @C –H, @C10–H);
7.75 (dd, J = 7.8 Hz, 1.6 Hz, 1H, @C –H); 7.59–7.55 (m, 1H, @C
H); 7.54–7.50 (m, 3H, @C –H, @C11–H, @C12–H); 7.07 (t,
J = 7.2 Hz, 1H, @C13–H); 6.96 (d, J = 8.4 Hz, 1H, @C14–H); 4.37 (s,
16 2 5
H N O Mo: C, 46.60; H, 3.88; N, 6.79.
1
Elemental analyses were performed on a Vario ELcube CHNS
6
, 400 MHz)
elemental analyzer. IR spectra were recorded on a Perkin-Elmer
7
2
Spectrum RXI spectrometer. 1H NMR spectra were recorded with
5
3
–
a Bruker Ultrashield 400 MHz spectrometer using SiMe
4
as an
4
internal standard. Electronic spectra were recorded on a Lamda25,
PerkinElmer spectrophotometer. Magnetic susceptibility was mea-
sured with a Sherwood Scientific AUTOMSB sample magnetometer.
Electrochemical data were collected using a PAR electrochemical
1H, –HO–C15); 3.40 (m, 2H, –C15–H); 1.04 (t, J = 7.2 Hz, 3H, –C16
–
1
3
H). C NMR (DMSO–d
6
, 100 MHz) d: 169.16, 159.83, 156.50,
135.42, 134.77, 132.46, 130.40, 129.28, 128.44, 122.08, 120.74,
119.03, 56.50, 19.01.
analyzer and
a PC-controlled Potentiostat/Galvanostat (PAR
2
73A) at 298 K in a dry nitrogen atmosphere. Cyclic voltammetry
experiments were carried out with a platinum working electrode,
platinum auxiliary electrode and Ag/AgCl as a reference electrode,
and TBAP as the supporting electrolyte.
2
2
.5. Synthesis of the mixed-ligand complexes [MoO L(Q)], where
Q = imidazole (2a), 1-methylimidazole (2b) and [(MoO
Q = 4,4 -bipyridine (3)
2 2
L) (Q)] where
0
2
.3. Synthesis of the ligand H
2
L
To
a clear orange solution (obtained by refluxing) of 1
The benzoylhydrazone ligand of 2-hydroxybenzaldehyde (H
was prepared by the condensation of an equimolar ratio of ben-
zoylhydrazide 2.72 g (20.00 mmol) and 2-hydroxybenzaldehyde
2
L)
(0.50 mmol) in ethanol (30 ml) was added the corresponding
mixed-ligand [0.73 mmol (2a and 2b), 0.30 mmol (3)] and the
mixture was refluxed for 3 h. Slow evaporation of the orange filtrate
over 2 days produced orange-red crystals. [MoO L(Imz)], (2a):
Yield 0.25 g (57%). Anal. Calc. for C17H14N O Mo: C, 47.11; H,
3.23; N, 12.93. Found: C, 47.07; H, 3.19; N, 12.89%. H NMR
(DMSO–d , 400 MHz) d: 8.95 (s, 1H, –N@C –H); 7.99 (d, J = 7.6 Hz,
2H, @C10–H, @C14–H); 7.74 (d, J = 7.6 Hz, 1H, @C17–H); 7.67 (br s,
1H, @N –H); 7.61–7.49 (m, 5H, @C –H, @C –H, @C11–H, @C12–H,
@C13–H); 7.09 (t, J = 7.6 Hz, 1H, @C16–H); 7.01 (s, 1H, @C15–H);
6.96 (d, J = 8.4 Hz, 2H, @C –H, @C –H).
1
.46 g (20.00 mmol) in stirring ethanol (25 ml) for 2 hr. The result-
ing yellowish-white compound was filtered, washed with ethanol
and dried over fused CaCl . M.P. 160 °C. Yield 2.88 g (61%). Anal.
Calc. for C14 : C, 70.00; H, 5.00; N, 11.66. Found: C, 70.08;
H, 5.03; N, 11.62%. H NMR (DMSO–d
2
4
4
1
2
H
12
N
2
O
2
6
7
1
6
, 400 MHz)d: 12.14 (s, 1H,
H); 8.66 (s, 1H, –N@C –H); 7.96–
–H, @C10–H); 7.63–7.60 (m, 1H, @C14–H); 7.56–
–H, @C –H, @C –H); 7.33–7.28 (m, 1H, @C13–H);
.96–6.91 (m, 2H, @C11–H, @C12–H).
–
7
7
6
1
1
C
1
–OH); 11.32 (s, 1H, –C
.94 (m, 2H, @C
.53 (m, 3H, @C
8
–N
2
7
4
3
4
2
3
4
5
5
2
13
C
NMR (DMSO–d
6
,
[MoO L(1-MeImz)], (2b): Yield 0.35 g (78%). Anal. Calc. for
C18H16N O Mo: C, 48.21; H, 3.57; N, 12.50. Found: C, 48.17; H,
3.54; N, 12.46%. H NMR (DMSO–d , 400 MHz) d: 8.96 (s, 1H,
6
2
00 MHz) d: 163.30, 157.94, 148.79, 133.27, 132.45, 131.86,
30.01, 129.02, 128.11, 119.82, 119.13, 116.89.
4
4
1
–
N@C
J = 1.6 Hz, 7.6 Hz, 1H, @C17–H); 7.61–7.49 (m, 5H, @C
11–H, @C12–H, @C13–H); 7.11–7.07 (m, 2H, @C –H, @C
5
7
–H); 7.99 (d, J = 7.2 Hz, 2H, @C10–H, @C14–H); 7.74 (dd,
–H, @C –H,
–H);
2
.4. Synthesis of the precursor complex [MoO
L(C
2 2
5
H OH)], (1)
3
4
@
C
2
0
.24 g (1.00 mmol) of the ligand H L was dissolved in 30 mL
2
6.96 (d, J = 7.6 Hz, 1H, @C16–H); 6.87 (s, 1H, @C15–H); 3.63(s, 3H,
ethanol. When 0.32 g (1.00 mmol) of MoO
2
(acac)
2
was added to
–C18–H).
Table 1
Crystal and refinement data for complexes 1, 2a, 2b and 3.
Compound
1
2a
2b
3
Formula
C
16
H
16 Mo N
2
O
5
C
17
H
14 Mo N
434.26
orthorhombic
Pna2
4
O
4
C
18
H
16 Mo N
448.29
orthorhombic
Pca2
4
O
4
C
38 2 6 8
H28 Mo N O
Formula weight
Crystal symmetry
Space group
Unit cell dimensions
a (Å)
412.25
monoclinic
P2
888.54
monoclinic
P2 /n
1
1
/c
1
1
10.4986(7)
10.4410(7)
15.1561(11)
106.842(2)
1590.09(19)
4
7.6276(14)
16.640(3)
13.260(3)
90
1683.0(5)
4
17.0374(5)
6.7818(2)
32.3366(9)
90
3736.31(19)
8
11.1975(3)
13.0914(4)
12.5884(3)
98.875(2)
1823.25(9)
2
b (Å)
c (Å)
b (°)
3
V (Å )
Z
D
calc (g cm 3
ꢀ
)
1.722
1.714
1.594
1.618
F(000)
(Mo-K
Maximum/minimum trans.
h (max) (°)
Reflections collected/unique
832
0.854
0.9505/0.8826
30.5
23567/4900
872
0.811
0.8047/0.7757
28
16063/3880
1808
0.733
0.8100/0.7939
30.50
68324/11367
880
0.749
0.8064/0.7794
30.20
15801/5495
) (mm 1
ꢀ
)
l
a
2
a
R
1
[I > 2
r
(I)]
[all data]
Goodness-of-fit (GOF) on F
Maximum/minimum res. (e Åꢀ3)
R
R
1
= 0.0281, wR
= 0.0324, wR
2
2
= 0.0671
= 0.0690
R
R
1
= 0.0360, wR = 0.0790
2
= 0.0455, wR = 0.0835
2
R
R
1
= 0.0413, wR
= 0.0716, wR
2
2
= 0.0817
= 0.0920
R
R
1
= 0.0611, wR
= 0.1310, wR
2
2
= 0.1431
= 0.1742
b
wR
2
1
1
1
1
2
1.079
0.779/ꢀ0.809
1.077
0.997/ꢀ0.518
1.012
0.413/ꢀ0.571
1.037
2.525/ꢀ0.696
aR
wR
=
R
= {
|F
R
| ꢀ |F
|/
R
|F
|.
1
o
c
o
b
2
2
2
[w(F ) ]}1/2.
2
2
2
[w(F ꢀ F ) ]/
o
c
R
o

2
00
S. Pasayat et al. / Polyhedron 38 (2012) 198–204
0
[
(MoO
2
L)
Mo
.10; N, 9.39%. H NMR (DMSO–d
N@C –H); 8.71 (d, J = 5.6 Hz, 2H, @C15–H, @C19–H); 7.99 (d,
2
(
l
2
-4,4 -bipy)], (3): Yield 0.54 g (60%). Anal. Calc. for
DMSO alone was used as a negative control under the same condi-
tions for each organism.
C
3
–
38
H
28
N
6
O
8
: C, 51.35; H, 3.15; N, 9.45. Found: C, 51.30; H,
1
6
, 400 MHz) d: 8.96 (s, 1H,
7
3
. Results and discussion
J = 7.2 Hz, 2H, @C10–H, @C14–H); 7.82 (dd, J = 1.2 Hz, 8.4 Hz,
H, @C16–H, @C18–H); 7.75–7.73 (m, 1H, @C –H); 7.58–7.49
m, 4H, @C –H, @C11–H, @C12–H, @C13–H); 7.087 (t, J = 7.6 Hz,
–H); 6.96 (d, J = 8.4 Hz, 1H, @C –H).
2
3
3
3
.1. Synthesis and structure
(
4
1
H, @C
2
5
.1.1. Description of the X-ray structure of complex 1
The reaction of the benzoylhydrazone of 2-hydroxybenzalde-
hyde (H L) with [MoO (acac) ] proceeds smoothly in refluxing eth-
2 2 2
2.6. Crystallography
anol to afford an orange complex in a decent yield. Although the
preliminary characterization data (microanalysis, H NMR and IR)
indicated the presence of the ligand, two oxo groups and ethanol
in this complex, they could not point to any definite stereochemis-
2
try of the complex, or the coordination mode of H L. For an unam-
Suitable single crystals of 1, 2a, 2b and 3 were chosen for X-ray
diffraction studies. Crystallographic data and details of refinement
are given in Table 1. These compounds crystallized in the mono-
1
clinic space group P2
1
/c (1), orthorhombic space group Pna2
2a), orthorhombic space group Pca2 (2b) and monoclinic space
/n (3). The unit cell parameters and the intensity data
1
(
1
biguous characterization of this complex, its structure was
determined by X-ray crystallography. The structure is shown in
Fig. 1 and some relevant bond parameters are listed in Table 2.
group P2
1
for all the complexes were collected at ꢁ293 K, on a Bruker Smart
Apex CCD diffractometer using graphite monochromated Mo-K
radiation (k = 0.71073 Å), employing the –2h scan technique.
a
+2
The structure shows that the ligand is coordinated to MoO
2
as
x
an O,N,O-donor (A), with bite angles of 81.08(5)° [N(1)–Mo(1)–
O(1)] and 71.11(5)° [O(2)–Mo(1)–N(1)]. One ethanol molecule is
also coordinated to the metal center. This complex is therefore for-
mulated as [MoO L(C H OH)] (1), where L refers to the ben-
2 2 5
zoylhydrazone ligand of 2-hydroxybenzaldehyde, coordinated as
The intensity data were corrected for Lorentz, polarization and
absorption effects. The structures were solved and refined using
the SHELXS-97 [43] and SHELXL-97 [44] computer programs, respec-
tively. The non-hydrogen atoms were refined anisotropically.
in A (Chart 1). The observed microanalytical data are also consis-
2.7. Antibacterial activity
tent with this composition. In this [MoO
2 2 5
L(C H OH)] complex,
molybdenum is sitting in an NO coordination sphere, which is sig-
5
The antibacterial activity of the compounds (ligands and com-
nificantly distorted from an ideal octahedral geometry as reflected
in the bond parameters around the metal center. The Mo@O dis-
tance is quite normal [4,23,28,29] and so are the Mo–O, Mo–N,
C@N and N–N bond lengths within the Mo(L) fragment. The
plexes) was studied against Escherichia coli, Bacillus and Pseudomo-
nas aeruginosa by the agar well diffusion technique. Mueller
Hinton-agar (containing 1% peptone, 0.6% yeast extract, 0.5% beef
extract and 0.5% NaCl, at pH 6.9–7.1) plates were prepared and
8
0
.5 – McFarland culture (1.5 ꢂ 10 cells/mL) of the test organisms
were swabbed onto the agar plate (as per CLSI guidelines, 2006).
Test compounds were dissolved in DMSO at several concentrations
Table 2
Selected bond distances (Å) and bond angles (°) for complex 1.
ranging from 1000 to 1.95
lg/mL. 9 mm wide wells were dug on
Distances
the agar plate using a sterile cork borer. 100
of each test compound from each dilution were added into each of
the wells using a micropipette. These plates were incubated for
l
g/mL of the solutions
Mo(1)–O(1)
Mo(1)–O(3)
Mo(1)–O(5)
1.928(1)
1.698(2)
2.362(2)
Mo(1)–O(2)
Mo(1)–O(4)
Mo(1)–N(1)
2.020(1)
1.702(1)
2.237(1)
2
4 h at 35 ± 2 °C. The growth of the test organisms was inhibited
Angles
O(1)–Mo(1)–O(2)
O(1)–Mo(1)–O(4)
O(1)–Mo(1)–N(1)
O(2)–Mo(1)–O(4)
O(2)–Mo(1)–N(1)
O(3)–Mo(1)–O(5)
O(4)–Mo(1)–O(5)
O(5)–Mo(1)–N(1)
149.09(5)
102.65(6)
81.08(5)
98.39(6)
71.11(5)
171.06(6)
82.41(6)
76.28(5)
O(1)–Mo(1)–O(3)
O(1)–Mo(1)–O(5)
O(2)–Mo(1)–O(3)
O(2)–Mo(1)–O(5)
O(3)–Mo(1)–O(4)
O(3)–Mo(1)–N(1)
O(4)–Mo(1)–N(1)
99.19(7)
82.08(5)
96.54(6)
78.51(5)
105.81(7)
95.12(7)
157.69(6)
by diffusion of the test compounds and then the inhibition zones
that developed on the plates were measured [41,45]. The effective-
ness of an antibacterial agent in sensitivity is based on the diame-
ter of the zones of inhibition, which was measured to the nearest
millimeter (mm). The standard drug Vancomycin was also tested
for its antibacterial activity at the same concentration under condi-
tions similar to that of the test compounds as a positive control.
2 2 5
Fig. 1. ORTEP diagram of [MoO L(C H OH)], 1, with the atom labeling scheme.

S. Pasayat et al. / Polyhedron 38 (2012) 198–204
201
H
C
3
2
H
O
H
4
1
5
H
6
O
7
1
O
Mo
N
H
1
3
6
15
H C H C OH
2
O
N
2
8
2
9
1
0
H
H
1
4
1
H
1
H
13
1
H
Chart 1. Schematic representation of the coordinated ligand, A.
2 5
Mo–O(C H OH) bond length in complex 1 is longer than the nor-
mal single bond length [2.362(2) Å against 1.928(1)–2.020(1) Å],
which indicates a labile site available for substitutions. This
possibility is realized in the facile formation of adducts of the
2
Fig. 2. ORTEP diagram of [MoO L(Imz)], 2a, with the atom labeling scheme.
2
general formula [MoO L(Q)] (2a and 2b).
3.1.2. X-ray structures of complexes 2a, 2b and 3
The formation of mixed-ligand mononuclear [MoO
2
L(Q)] com-
plexes have been achieved by the reaction of 1 with different Lewis
bases {Q = imidazole (2a) and 1-methylimidazole (2b)} in excellent
yield. The formation of the dinuclear dioxomolybdenum(VI) com-
0
plex [(MoO
1
2
L)
2
(l
-4,4 -bipy)] (3) was achieved by the reaction of
0
and 4,4 -bipyridine as an exo-bidentate ligand under refluxing
conditions. The common characteristic of these structures is the
absence of a metal-coordinating solvent molecule, and its replace-
ment by heterocyclic bases, bonding through a nitrogen atom. The
atom numbering schemes for complexes 2a and 2b are given in
Figs. 2 and 3, respectively, with the relevant bond distances and
angles collected in Tables 3 and 4, respectively. The molecular
structures of the complexes [MoO
mz)] (2b) have shown that the dinegative hydrazone ligand (H
2
L(Imz)] (2a) and [MoO
2
L(1-MeI-
L)
2
bind to the molybdenum(VI) center by its O(1), N(1) and O(2) do-
nor atoms. The fourth coordination site around molybdenum(VI) is
occupied by an imidazole ligand in complex 2a and by 1-methyl-
imidazole in 2b through its tertiary nitrogen N(3), forming dis-
torted octahedral complexes. The rather large Mo(1)–N(3)
distances are 2.348(4) (2a) and 2.353(3) Å (2b), which reveal that
the imidazole and 1-methylimidazole moieties are also rather
Fig. 3. ORTEP diagram of [MoO
2
L(1-MeImz)], 2b, with the atom labeling scheme.
2+
weakly coordinated to the MoO
2
core [4,23,28,29]. The Mo–O
2+
0
(
oxo) bond distances of the MoO
2
group are unexceptional
complex [(MoO
2
L)
2
(l
-4,4 -bipy)] closely resembles the structure
[
23,29] and are almost equal [1.695(2)-1.706(3) Å] for both com-
of the other half of the complex. The coordination environment
around the Mo(VI) center is octahedral and highly distorted. The li-
gand H L is dianionic and tridentate, with its meridionally situated
2
2
+
plexes (2a and 2b). The ligand coordinates to the MoO
the deprotonated enolate form because in complexes 1, 2a and
2
core in
2
1
b the C–O bond distances [C(8)–O(2)] exhibit values of 1.311(2),
.335(4) and 1.325(5) Å, respectively, which are nearer to a C–O
donor sites O(1), N(1) and O(2) lying in the equatorial plane along
with the oxo oxygen O(4). The bond distances about Mo(1) reveal
the magnitude of the distortions, as can be seen in Table 5. The
Mo–O distances range from 1.688(4) Å for the oxo ligand O(3), lo-
cated in an axial position, to 1.990(3) Å for the enolate oxygen
O(2). The Mo(1)–O(1) (phenolate) distance is slightly shorter at
1.923(4) Å. The second axial position is occupied by a nitrogen
single bond than to a C–O double bond distance. However, it falls
short of a pure C–O single bond distance of 1.42 Å because of delo-
calization of electrons.
The dinuclear complex 3 (Fig. 4, Table 5) crystallizes in the
monoclinic space group P2
crystallographic center of inversion. Each half of the dinuclear
1
/n with the molecule sitting across a
0
atom of the bridging 4,4 -bipyridine ligand, which is significantly

2
02
S. Pasayat et al. / Polyhedron 38 (2012) 198–204
Table 3
Table 5
Selected bond distances (Å) and bond angles (°) for complex 2a.
Selected bond distances (Å) and bond angles (°) for complex 3.
Distances
Distances
Mo(1)–O(1)
Mo(1)–O(3)
Mo(1)–N(1)
1.929(4)
1.697(3)
2.251(3)
Mo(1)–O(2)
Mo(1)–O(4)
Mo(1)–N(3)
2.028(3)
1.695(2)
2.348(4)
Mo(1)–O(1)
Mo(1)–O(3)
Mo(1)–N(1)
1.923(4)
1.688(4)
2.249(4)
Mo(1)–O(2)
Mo(1)–O(4)
Mo(1)–N(3)
1.990(3)
1.703(4)
2.487(4)
Angles
Angles
O(1)–Mo(1)–O(2)
O(1)–Mo(1)–O(4)
O(1)–Mo(1)–N(3)
O(2)–Mo(1)–O(4)
O(2)–Mo(1)–N(3)
O(3)–Mo(1)–N(1)
O(4)–Mo(1)–N(1)
N(1)–Mo(1)–N(3)
150.0(1)
104.4(1)
83.7(1)
97.5(1)
78.5(1)
92.4(1)
160.7(1)
78.3(1)
O(1)–Mo(1)–O(3)
O(1)–Mo(1)–N(1)
O(2)–Mo(1)–O(3)
O(2)–Mo(1)–N(1)
O(3)–Mo(1)–O(4)
O(3)–Mo(1)–N(3)
O(4)–Mo(1)–N(3)
100.8(2)
81.4(1)
93.2(1)
71.6(1)
104.3(2)
169.0(1)
84.0(1)
O(1)–Mo(1)–O(2)
O(1)–Mo(1)–O(4)
O(1)–Mo(1)–N(3)
O(2)–Mo(1)–O(4)
O(2)–Mo(1)–N(3)
O(3)–Mo(1)–N(1)
O(4)–Mo(1)–N(1)
N(1)–Mo(1)–N(3)
148.5(1)
104.2(2)
80.7(1)
95.5(2)
77.8(1)
94.6(2)
157.2(2)
6.8(1)
O(1)–Mo(1)–O(3)
O(1)–Mo(1)–N(1)
O(2)–Mo(1)–O(3)
O(2)–Mo(1)–N(1)
O(3)–Mo(1)–O(4)
O(3)–Mo(1)–N(3)
O(4)–Mo(1)–N(3)
98.8(2)
81.2(1)
99.0(2)
71.61(1)
106.2(2)
171.3(2)
82.2(2)
3.2. Spectral characteristics
Table 4
The spectral characteristics of compounds 1–3 are listed in
Selected bond distances (Å) and bond angles (°) for complex 2b.
Table 6. A number of absorption bands are observed in the
UV–Vis region. The DMF solutions of all the complexes display a
shoulder in the 410–402 nm region and two strong absorptions
are located in the 316–310 and 269–219 nm range, which
Distances
Mo(1)–O(1)
Mo(1)–O(3)
Mo(1)–N(1)
1.905(3)
1.703(3)
2.239(3)
Mo(1)–O(2)
Mo(1)–O(4)
Mo(1)–N(3)
2.010(3)
1.706(3)
2.353(3)
are assignable to L–Mo(dp) LMCT and intraligand transitions,
Angles
respectively.[23,28,29]
O(1)–Mo(1)–O(2)
O(1)–Mo(1)–O(4)
O(1)–Mo(1)–N(3)
O(2)–Mo(1)–O(4)
O(2)–Mo(1)–N(3)
O(3)–Mo(1)–N(1)
O(4)–Mo(1)–N(1)
N(1)–Mo(1)–N(3)
150.4(1)
103.9(1)
81.5(1)
96.7(1)
79.6(1)
91.0(1)
160.4(1)
78.2(1)
O(1)–Mo(1)–O(3)
O(1)–Mo(1)–N(1)
O(2)–Mo(1)–O(3)
O(2)–Mo(1)–N(1)
O(3)–Mo(1)–O(4)
O(3)–Mo(1)–N(3)
O(4)–Mo(1)–N(3)
100.2(1)
82.1(1)
94.3(1)
71.9(1)
106.0(1)
168.8(1)
84.2(1)
The infrared spectra of all the complexes are mostly similar. The
complexes do not exhibit the ligand bands at 3330 [
m
(OH)], 3265
ꢀ1
[
m
(NH)] and 1672 cm
[
m
(C@O)] [27–33]. Characteristic strong
ꢀ1
bands at ꢁ1605 and ꢁ1539 cm due to
m(C@N) and m(C@C/aro-
matic) stretching modes of the ligand [23,29] are located in the
spectra of both the ligand and the complexes. The Mo@O stretch-
ing modes occur as a pair of sharp strong peaks in the 937–
ꢀ
1
8
92 cm range [23,25–29].
Magnetic susceptibility measurements show that all the com-
further from the Mo center than the other five ligated atoms.
Mo(1)–N(3), at 2.487(4) Å, is undoubtedly the weakest of the six
Mo–L bonds and consequently the most susceptible to ligand ex-
change [29]. The chelate bite angles for the five- and six-membered
rings have values within the expected ranges [O(2)–Mo(1)–N(1),
plexes (1–3) are diamagnetic, which corresponds to the +6 oxida-
1
tion state of molybdenum. The H NMR spectrum of the free
ligand exhibits an OH (phenolic) resonance at d = 12.14 ppm, an
NH resonance at d = 11.32 ppm and
a
CH resonance at
d = 8.66 ppm. All the aromatic proton signals expected from it are
7
1.61(1)°; O(1)–Mo(1)–N(1), 81.2(1)°].
0
2 2
Fig. 4. ORTEP diagram of [(MoO L) (l-4,4 -bipy)], 3, with the atom labeling scheme.

S. Pasayat et al. / Polyhedron 38 (2012) 198–204
203
Table 6
Characteristic IR bands and electronic spectral datab for the studied complexes.
a
(Mo@O) (cm 1
ꢀ
)
kmax (nm [e
/dm3 mol cm-1])
ꢀ1
Complex
m
[
[
[
[
MoO
MoO
MoO
2
2
2
L (C
L(Imz)], 2a
L(1-MeImz)], 2b
2
H
5
OH)], 1
937, 913
935, 892
912,898
932, 910
402 (9439), 316 (25123), 219 (16352)
403 (9569), 316 (26126), 220 (15711)
410 (10801), 315 (23470), 266 (16352)
405 (6393), 310 (22768), 269 (16939)
0
(MoO
2
L)
2
(
l-4,4 -bipy)], 3
a
In KBr pellet.
In DMF
b
Table 7
Table 8
Minimum inhibitory concentration (MIC) value in
(H
2
Cyclic voltammetric results for dioxomolybdenum(VI)
complexes at 298 K.
l
g/mL of the Schiff base ligand
L ), oxomolybdenum(VI) complexes and standard drugs against pathogenic
strains.
1
pc (V)a
E
Complex
[
[
[
[
MoO
MoO
MoO
2
2
2
L (C
L(Imz)], 2a
L(1-MeImz)], 2b
2
H
5
OH)], 1
ꢀ0.81
E. coli
Bacillus
P. aeruginosa
ꢀ0.71, ꢀ0.92
ꢀ0.71, ꢀ1.00
H
2
L
125
125
15.6
15.6
125
30
500
62.5
15.6
125
–
–
–
1000
1000
500
–
[
[
MoO
MoO
2
2
2
L(C
L(Imz)] (2a)
L(1-MeImz)](2b)
2
H
5
OH)] (1)
0
(MoO
2
L)
2
(
l-4,4 -bipy)], 3 ꢀ0.79, ꢀ1.01, ꢀ1.61
a
[MoO
Solvent: DMF; working electrode: platinum; aux-
0
[
(MoO
2
L)
2
(l
-4,4 -bipy)] (3)
iliary electrode: platinum; reference electrode: Ag/
AgCl; supporting electrolyte: 0.1 M TBAP; scan rate:
Vancomycin
30
50 mV/s. Epc is the cathodic peak potential.
possible explanation is that by coordination, the polarity of the
ligand and the central metal ion are reduced through charge
equilibration, which favors permeation of the complexes through
the lipid layer of the bacterial cell membrane [16,41,45,47–49].
From the zone of inhibition, it is observed that complexes 2a
and 2b show the most promising results as compared to the other
compounds (1 and 3) of this study and the difference in value may
be attributed to the nature of compounds synthesized with imida-
zoles as co-ligands [50,51]. The antibacterial activity of a similar
type of oxo-metal complex has also been reported by Sharma
et al. [52], Chohan et al. [53] and Prasad et al. [54] using the agar
well diffusion technique against E. coli, Shigella, Pseudomonas,
Salmonella, Staphylococcus and Bacillus, and the present results
are in accordance with the reported values. However, the differ-
ence in value may be attributed to the nature of compounds
synthesized with different ligands.
clearly observed in the expected region. In the complexes 1–3, the
signals for the OH and NH protons have disappeared, indicating the
deprotonation of the phenolic OH and NH groups in the complexes
[
23–29]. The chemical shift of the coordinated EtOH of complex 1
exhibits an OH (enolic) resonance at d = 4.37 ppm, CH resonance
at d = 3.40 ppm and CH resonance at d = 1.04 ppm. These data
2
3
are different compared to the chemical shifts found [46] for free
molecule at 4.63, 3.44 and 1.06 ppm, respectively. The 13C signals
expected from the coordinated EtOH are clearly observed in the ex-
pected region. Therefore, the NMR data of complex 1 clearly indi-
cate that the labile solvent molecule, i.e. EtOH, stays coordinated
during the solution study. The NMR spectroscopic data of all the
complexes are given in the experimental section.
3.3. Electrochemical properties
Electrochemical properties of the complexes have been studied
by cyclic voltammetry in DMF solution (0.1 M TBAP), and the vol-
tammetric data are given in Table 7. The CV trace of complex 1
exhibits one irreversible reductive response at ꢀ0.81 V, which is
4. Conclusion
In summary, this article presents the synthesis and structures of
a mononuclear Schiff base complex 1, some mixed-ligand mononu-
clear (2a and 2b) and diimine bridged dinuclear (3) oxomolybde-
num(VI) complexes, and sheds some light on their antibacterial
activity. The coordinated ethanol in the oxomolybdenum(VI) com-
VI
V
assigned to the Mo /Mo process [23], but the CV traces of com-
plexes 2a and 2b exhibit two irreversible reductive responses
within the potential window ꢀ0.71 to ꢀ1.0 V, which are assigned
VI
V
V
IV
to Mo /Mo and Mo /Mo processes, respectively [28].The dinu-
clear complex 3 exhibits three successive irreversible one-electron
reductions within the potential window ꢀ0.79 to ꢀ1.61 V versus
2 2 5
plex [MoO L(C H OH)] (1) with aroylhydrazone is found to act as a
substrate-binding site, and mixed-ligand six-coordinate adducts,
VI
VI
V
VI
Ag/AgCl in DMF, which are assigned to Mo –Mo /Mo –Mo and
like [MoO
2
L(Imz)] (2a), [MoO
2
L(1-MeImz)] (2b) and [(MoO
2
L)
2
V
VI
V
V
0
Mo –Mo /Mo –Mo processes and the ligand (bridging)-centered
reduction, respectively [29].
(l-4,4 -bipy)] (3) are easily formed by the attachment of imidazole,
0
1-methylimidazole and 4,4 -bipyridine to this position as a co-li-
gand, respectively. The complexes have been screened for their
antibacterial activity against E. coli, Bacillus and P. aeruginosa. The
minimum inhibitory concentration of these complexes and their
antibacterial activities indicate that compounds 2a and 2b are po-
tential lead molecules for drug design.
3
.4. Antimicrobial activity
The in vitro antimicrobial activity of the synthesized oxomolyb-
denum(VI) complexes 1–3 have been tested against the pathogenic
strains of E. coli, Bacillus and P. aeruginosa by the agar well-diffu-
sion method and, in some cases, they showed promising results
by giving MIC values lower than the standard drug examined.
The results (Table 8) also indicate that the corresponding oxomo-
lybdenum(VI) complexes showed much better antibacterial activ-
ity with respect to the individual ligand against the same
microorganism under identical experimental conditions, which
is in agreement with the previous results [16,41,45,47–49]. A
Acknowledgements
The authors thank the reviewers for their comments and sug-
gestions, which were helpful in preparing the revised version
of the manuscript. The authors thank Dr. Debayan Sarkar, NIT,
Rourkela for fruitful discussion. We also acknowledge the use of
the UV–Vis spectrophotometer purchased from the DST (India)

2
04
S. Pasayat et al. / Polyhedron 38 (2012) 198–204
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acknowledges the Council of Scientific and Industrial Research,
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