Supramolecular frameworks of binuclear dioxomolybdenum(vi) complexes with ONS donor ligands using 4,4′-azopyridine as a pillar: crystal structure, DFT calculations and biological study

Article abstract of DOI:10.1039/c5nj01899d

Three new isostructural pillared binuclear dioxomolybdenum(vi) complexes [(MoO₂L¹)₂(4,4′-azpy)] (1), [(MoO₂L²)₂(4,4′-azpy)] (2) and [(MoO₂L³)₂(4,4′-azpy)] (

Full text of DOI:10.1039/c5nj01899d

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Supramolecular frameworks of binuclear
dioxomolybdenum(VI) complexes with ONS donor
ligands using 4,40-azopyridine as a pillar: crystal
structure, DFT calculations and biological study†
Cite this:DOI: 10.1039/c5nj01899d
Debanjana Biswal,a Nikhil Ranjan Pramanik,*b Syamal Chakrabarti,*a
Michael G. B. Drew,c Payel Mitra,d Krishnendu Acharya,d Sujan Biswase and
Tapan Kumar Mondale
Three new isostructural pillared binuclear dioxomolybdenum(VI) complexes [(MoO2L1)2(4,40-azpy)] (1),
[(MoO2L2)2(4,40-azpy)] (2) and [(MoO2L3)2(4,40-azpy)] (3) [where, 4,40-azpy = 4,40-azobis-(pyridine)]
containing tridentate binegative Schiff base ligands (H2L1, H2L2 and H2L3) obtained by the condensation
of salicylaldehyde and substituted salicylaldehyde with S-benzyl dithiocarbazate have been reported for
the first time. These complexes have been characterized by spectroscopic and electrochemical studies.
In all the complexes (1–3), 4,40-azpy behaves as an auxiliary spacer and bridges the Mo(VI) centers giving
rise to neutral 3D supramolecular frameworks. The cavities formed by hydrogen bonding interactions
are capable of hosting guest molecules. The crystal structures reveal that each molybdenum site in the
binuclear complexes adopts a distorted octahedral geometry. The morphology and structures of the
complexes are further examined by SEM. Thermal behavior has been investigated to study the framework
stabilities of these structures. Supportive DFT calculations on the complexes have been carried out. These
ligands and complexes 1–3 have significant antimicrobial and antioxidant properties.
Received (in Victoria, Australia)
20th July 2015,
Accepted 26th August 2015
DOI: 10.1039/c5nj01899d
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complexes are used as effective catalysts in industry12–14 and
oxidation reactions15 because they possess a large number of
Introduction
Molybdenum chemistry has become a prospective area of research stable and accessible oxidation states as well as coordination
because of the presence of molybdenum in metalloenzymes and its numbers. Numerous mononuclear dioxomolybdenum(VI) complexes
significant role in hydroxylase and oxo-transferase enzymes1–7 have been reported but the crystal structures of few binuclear
and due to the possible antifungal,8 antibacterial,9 antitumor10 and Mo(VI) complexes with ONS donor ligands have been determined
antiviral11 properties of molybdenum complexes. Molybdenum so far.16–19 The construction of coordination complexes with new
network motifs is of current interest for the development of
functional materials and in fundamental studies of crystal
a Department of Chemistry, University College of Science,
engineering and supramolecular chemistry.16,20–22 Supramolecular
frameworks involving organic bridging ligands23 acting as spacers
have been an interesting area of research. Hydrogen bonding
92, Acharya Prafulla Chandra Road, Kolkata-700009, West Bengal, India.
E-mail: schakrabarti2014@gmail.com; Fax: +91 033-2351-9755;
Tel: +91 033-2350-8386
b Department of Chemistry, Bidhannagar College, EB-2,Sector-1, Salt Lake,
Kolkata-700064, India. E-mail: nr_pramanik@yahoo.co.in;
Fax: +91 033-2337-4782; Tel: +91 033-2337-4389
plays a significant role in supramolecular assemblies with
structural diversity and potential applications in various fields
such as selective gas-adsorption,24 separation,25 drug delivery26
and catalysis27 because of its dimensional nature and strength.
Pyridine and its derivatives have been very useful mono-
dentate ligands in many aspects of coordination chemistry.28
c Department of Chemistry, The University of Reading, Whiteknights,
Reading RG66AD, UK
d Molecular and Applied Mycology and Plant Pathology Laboratory,
Department of Botany, University of Calcutta, Kolkata 700019, West Bengal, India
e Department of Chemistry (Inorganic section), Jadavpur University, Kolkata 700032, Bipyridyl bridging ligands incorporating alkane, alkene and
India
alkyne functionalities have been widely used for the construction of
binuclear complexes.16,29,30 Neutral N-donor ligands are often used
† Electronic supplementary information (ESI) available: Fig. S1–S8 contain cyclic
voltammograms, TG–DT curves, various supramolecular architectures, and
as auxiliary ligands owing to their affinities to transition metal ions
Tables S1–S3 contain the energy and composition of selected molecular orbitals
and conformations as pillar-like connectors.31 These auxiliary
of complexes 1–3. CCDC 1054805, 1054806 and 1054804. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/c5nj01899d
ligands play an important role in controlling the topology and
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dimensionality of the resultant supramolecular frameworks
containing cavities or channels which are crucial for thermal
stability. However, the coordination properties of closely related
bipyridyl ligands comprising the azo (–NQN–) functional moiety,
namely 4,40-azopyridine have received less attention. Therefore, the
construction of complexes containing 4,40-azpy as an auxiliary
ligand should be very interesting from the structural viewpoint.
A search of the Cambridge Crystallographic Database shows
that complexes involving the 4,40-azpy spacer have been
reported previously with many transition metals,32 but not so
far including molybdenum. It is to be noted that no literature
examples of the use of 4,40-azpy as a pillaring spacer with
[MoO2L] (L = ONS donor Schiff base ligand) in supramolecular
framework syntheses exist. Herein, we report for the first time
the 4,40-azpy spacer for the fabrication of three new isostructural
dioxomolybdenum(VI) binuclear pillar complexes 1–3 with ONS
donor ligands. In these complexes non-covalent interactions play
a pivotal role in the construction of fascinating neutral 3D
supramolecular networks. The 4,40-azpy spacer is particularly
useful in that because it can provide enough space to create
cavities. The presence of interaction sites on the pore surface
enhances guest anchoring. The solid state molecular structures
of complexes 1–3 have been ascertained by single-crystal X-ray
diffraction methods. Each Mo site in the binuclear complexes
adopts an octahedral geometry. The structures contain two
[MoO2]2+ moieties bridged by two nitrogen atoms from the
4,40-azpy spacer. The details of the crystal structures, host–guest
interactions, and framework stability are discussed later. The
morphology of the complexes is examined by SEM. Their thermal
and electrochemical behaviors have been investigated. Supportive
DFT calculations on the complexes have been carried out in order
to obtain deeper insight into the electronic situation around the
molybdenum centre. The significant antimicrobial and antioxidant
activities of these ligands and investigated complexes have also
been examined.
Scheme 1 Reaction diagram for isolation of the ligands.
59.71; H, 4.80; N, 9.43, IR (KBr Pellet), cmꢀ1: n(O–H) 3425(m), n(N–H)
1
3100(m), n(CQN) 1617(s), 1604(s), n(CQS) 1311(s); H NMR(CDCl3):
(–S–CH2–) 4.59 s (2H), (aromatic protons) 6.92–7.43 m (9H),
(H–CQN) 8.04 s (1H), (–NH–) 10.02 s (1H), (aromatic –OH)
10.72 s (1H).
S-Benzyl-b-N-(5-nitro-2-hydroxyphenyl)methylenedithiocarbazate
(H2L2). Anal. calc. for C15H13S2N3O3 (%): C, 51.87; H, 3.74; N, 12.10,
found: C, 51.65; H, 3.70; N, 12.00, IR (KBr Pellet), cmꢀ1: n(O–H)
3081(m), n(N–H) 2952(m), n(CQN) 1626(s), 1602(s), n(CQS) 1339(vs);
1H NMR (CDCl3): (–S–CH2–) 4.62 s (2H), (aromatic protons) 7.08–
7.43 m (8H), (H–CQN) 8.06 s (1H), (–NH–) 10.41 s (1H),
(aromatic –OH) 10.81 s (1H).
S-Benzyl-b-N-(5-bromo-2-hydroxyphenyl)methylenedithiocarbazate
(H2L3). Anal. calc. for C15H13S2N2OBr (%): C, 47.24; H, 3.41; N,
7.34, found: C, 47.10; H, 3.35; N, 7.28, IR (KBr Pellet), cmꢀ1
:
n(O–H) 3102(m), n(N–H) 2973(m), n(CQN) 1615(s), 1603(s), n(CQS)
1324(vs); 1H NMR (CDCl3): (–S–CH2–) 4.60 s (2H), (aromatic
protons) 6.88–7.49 m (8H), (H–CQN) 7.91 s (1H), (–NH–)
10.25 s (1H), (aromatic –OH) 11.28 s (1H).
Experimental section
Materials
Synthesis of the complexes
Reagent grade solvents were dried and distilled prior to use.
4,40-Azobis-(pyridine) (4,40-azpy) was prepared as an orange solid
by the procedures reported previously.33 All other chemicals
used for the preparative work were of reagent grade, available
commercially and used without further purification.
All the mononuclear MoO2L complexes were prepared by
refluxing MoO2(acac)2 and the respective ligands in dry chloro-
form for 2 h. The complexes were satisfactorily characterized by
1
elemental analyses, IR and H NMR.34
[(MoO2L1)2(4,40-azpy)]ꢁ2CHCl3 (1). A mixture of 0.856 g
(2 mmol) of MoO2L and 0.184 g (1 mmol) of 4,40-azobis-(pyridine)
in dry CHCl3 (20 mL) was refluxed for 3 h. The solution was
filtered. Shiny orange-red single crystals suitable for X-ray
diffraction analysis were obtained after one day. Yield B70%.
Synthesis
Synthesis of the ligands
The Schiff base ligands H2L1, H2L2 and H2L3 were prepared by Anal. calc. for C42H34S4N8O6Cl6Mo2 (%): C, 39.41; H, 2.65; N,
condensing S-benzyl dithiocarbazate with salicylaldehyde, 5-nitro- 8.75; Mo, 15.01, found: C, 39.35; H, 2.54; N, 8.69; Mo, 14.92, IR
salicylaldehyde and 5-bromosalicylaldehyde in ethanol by methods (KBr Pellet), cmꢀ1: n(CQN) 1599(vs), n(MoQO) 930(vs), 900(s), n(Mo–N)
reported previously34 (Scheme 1). The ligands were satisfactorily 635(s), n(Mo–S) 430(m); UV-Vis (CH2Cl2) [lmax/nm (e/dm3 molꢀ1
characterized by elemental analyses, IR and 1H NMR.
cmꢀ1)]: 299 (5740), 403 (1710); 1H NMR (CDCl3): (–S–CH2–Ph)
S-Benzyl-b-N-(2-hydroxyphenyl)methylenedithiocarbazate (H2L1). 4.43 s (4H), (aromatic protons) 7.22–7.68 m (26H), (H–CQN)
Anal. calc. for C15H14S2N2O (%): C, 59.60; H, 4.64; N, 9.30, found: C, 9.09 s (2H).
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Scheme 2 Reaction diagram for the synthesis of (MoO2L)2(4,40-azpy) complexes.
Complexes 2 and 3 were prepared similarly by refluxing the analysis for 1 and 3 were carried out using SAINT35 and for 2
corresponding parent MoO2L complex with 4,40-azopyridine using the CrysAlis program.36 The crystal structures were solved
(4,40-azpy) in a 2 : 1 molar ratio in dry CHCl3 for 3 h. (Scheme 2). by direct methods using SHELXS-9737 and refined by full-matrix
The solution was filtered. Shiny orange-red single crystals least-squares based on F2 with anisotropic displacement parameters
suitable for X-ray diffraction analysis were obtained after one of non-hydrogen atoms using SHELXL-97.37 The hydrogen atoms
day. Yield B70–75%.
were included in calculated positions. All three structures contained
[(MoO2L2)2(4,40-azpy)]ꢁ2CHCl3 (2). Anal. calc. for C42H32S4N10- solvent chloroform disordered over two superimposed sites. These
O10Cl6Mo2 (%): C, 36.81; H, 2.33; N, 10.22; Mo, 14.02, found: C, molecules were necessarily refined with distance constraints.
36.75; H, 2.30; N, 10.18; Mo, 14.00, IR (KBr Pellet), cmꢀ1: n(CQN) Thermal parameters in the bridging ligand in 3 were high but no
1597(vs), n(MoQO) 935(vs), 902(s), n(Mo–N) 664(s), n(Mo–S) 446(m); pattern of disorder could be identified. Absorption corrections
UV-Vis (CH2Cl2) [lmax/nm (e/dm3 molꢀ1 cmꢀ1)]: 294 (8730), 404 for 1 and 3 were carried out using SADABS38 and for 2 using
(1270); 1H NMR (CDCl3): (–S–CH2–Ph) 4.47 s (4H), (aromatic ABSPACK.39 The MERCURY40 and DIAMOND41 programs were
protons) 7.28–7.73 m (24H), (H–CQN) 8.85 s (2H).
used for the presentation of the structures.
[(MoO2L3)2(4,40-azpy)]ꢁ2CHCl3 (3). Anal. calc. for C42H32S4N8-
O6Cl6Br2Mo2 (%): C, 35.07; H, 2.22; N, 7.79; Mo, 13.36, found: C,
34.89; H, 2.17; N, 7.72; Mo, 13.01, IR (KBr Pellet), cmꢀ1: n(CQN)
1593(vs), n(MoQO) 932(vs), 901(s), n(Mo–N) 659(s), n(Mo–S) 435(m);
UV-Vis (CH2Cl2) [lmax/nm (e/dm3 molꢀ1 cmꢀ1)]: 298 (9110), 353
(4210); 1H NMR (CDCl3): (–S–CH2–Ph) 4.42 s (4H), (aromatic
protons) 7.28–7.75 m (24H), (H–CQN) 8.84 s (2H).
Computational details
Full geometry optimization of 1, 2 and 3 were carried out using
the DFT method at the B3LYP level of theory.42,43 The LanL2DZ
basis set with an effective core potential was employed for the
molybdenum atom.44–46 The 6-31+G(d) basis set was assigned
for all other elements. The vibrational frequency calculations
were performed to ensure that the optimized geometries represent
the local minima on the potential energy surface with only positive
Physical measurements
Elemental analyses were performed on a Perkin-Elmer 240 C, eigen-values. The lowest 50 singlet–singlet vertical electronic
H, N analyzer. NMR spectra were recorded on a Bruker 300 L excitations based on B3LYP optimized geometries were computed
NMR spectrometer operating at 300 MHz using TMS as internal using the time-dependent density functional theory (TDDFT)
standard. IR spectra were recorded as KBr pellets on a Perkin- formalism47–49 in CH2Cl2 using a conductor-like polarizable
Elmer model 883 infrared spectrophotometer. Electronic spectra continuum model (CPCM)50–52 using the same methodology.
were recorded using a HITACHI U-3501 UV-Vis recording spectro- All computations were performed using the Gaussian09 (G03)
photometer. Magnetic susceptibility was measured on a PAR model program.53 GaussSum54 was used to calculate the fractional
155 vibrating sample magnetometer using Hg[Co(SCN)4] as a contributions of various groups to each molecular orbital.
calibrant. Electrochemical data were collected on a Sycopel model
Antimicrobial activity
AEW2 1820 F/S instrument at 298 K using a Pt working electrode,
a Pt auxiliary electrode and a SCE reference electrode. Cyclic
voltammograms were recorded in DMF containing 0.1 M TBAP
as a supporting electrolyte. Thermal analyses were carried out on
a NETZSCH STA 449 F3 Jupiter thermal analyzer in a dynamic
atmosphere of dinitrogen (flow rate = 30 cm3 minꢀ1). SEM
images of the complexes were collected using an EVO 50 scanning
electron microscope from ZEISS. The operating voltage and
magnification were kept at 20 kV and 10 000ꢂ, respectively
throughout the study.
Test organisms. All the test human pathogenic microorganisms
Escherichia coli MTCC CODE 68, Bacillus subtilis MTCC CODE 736,
Listeria monocytogenes MTCC 657, Staphylococcus aureus MTCC 96
were obtained from the culture collection of the Microbial Type
Culture Collection and Gene Bank (MTCC), Institute of Microbial
Technology, Chandigarh, India.
Screening of antimicrobial activity by the disk diffusion
method. Antimicrobial activities of the ligands and complexes
1–3 were determined by the agar-disk diffusion method.55
Listeria monocytogenes MTCC 657 and Bacillus subtilis MTCC
121 were incubated at 28 1C for 1 day by inoculation in nutrient
Crystallographic measurements
The crystallographic data for complexes 1 and 3 were collected broth. Escherichia coli MTCC 1667 and Staphylococcus aureus
on a Bruker APEX-II diffractometer equipped with a CCD-area MTCC 96 were incubated at 37 1C. Nutrient agar was poured
detector, and for complex 2 was collected on an Oxford Diffrac- into each sterilized Petri dish (90 mm diameter) after injecting
tion X-Calibur System. All data sets were obtained with cultures (100 ml) of bacteria (106 cfu mlꢀ1) medium were
graphite-monochromated Mo-Ka (l = 0.71073 Å) radiation. distributed homogeneously. For the investigation of the antibacterial
Unit-cell dimensions and intensity data for 1, 2 and 3 were activity, filter paper disks (6 mm in diameter) were impregnated
measured at 296(2), 150(2) and 296(2) K, respectively. Data with individual compounds dissolved in 50% DMSO in order to
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reach the final levels of 10 mg per disk. The impregnated discs corresponding parent MoO2L complex with 4,40-azpy in a 2 : 1
were air dried before placing them on the Petri dishes with the molar ratio in dry chloroform.
test microorganisms and the plates were incubated at 37 1C.
All the orange-red binuclear dioxomolybdenum(VI) complexes
Discs with only 50% DMSO were used as negative controls. (1–3) are air stable in the solid state. The compounds are readily
Standard antibiotic discs with tetracycline (10 mg per disc) and soluble in alcohol, CH2Cl2, CH3CN, DMF and DMSO but are
ampicillin (10 mg per disc) were used as positive controls. The insoluble in water. The complexes are diamagnetic at room tem-
mean values obtained for three individual replicates were used perature as expected for d0Mo(VI) centers.58 Molar conductivity data
to calculate the zone of growth inhibition which was measured in 10ꢀ3 M CH2Cl2 solution indicate that they are non-electrolytes.
in mm using clear a graduated ruler. Studies were performed in All the binuclear Mo(VI) complexes are satisfactorily characterized
triplicate.
by elemental analyses, IR, 1H NMR, electronic spectra and
Determination of minimal inhibitory concentration (MIC). cyclic voltammetry. The complexes 1–3 have been structurally
The MIC study was aimed to find out the lowest concentration characterized by single crystal X-ray diffraction.
of the sample that inhibits the growth of the test organisms.
The test was carried out using the filter paper disc method with
1
IR and H NMR spectra
varying concentrations of the synthesized compounds placed Characteristic IR bands of the ligands and the corresponding
on bacteria. The plates were incubated for 24 h at 37 ꢃ 0.1 1C binuclear molybdenum(VI) complexes (1–3) are given in the
and 28 ꢃ 0.1 1C as required for the pathogens and the MIC Experimental section. The ligands (H2L1, H2L2 and H2L3)
recorded as the lowest concentration at which no growth was exhibit two broad bands of medium intensity in the 3425–
observed.
3081 cmꢀ1 range due to the n(O–H) mode and the 3100–2952 cmꢀ1
range is assigned to the n(N–H) vibration.59 Strong n(CQN) bands60,61 at
around 1620 cmꢀ1 of the free ligands are red shifted to 1599 cmꢀ1
,
Antioxidant activity
1597 cmꢀ1 and 1593 cmꢀ1, respectively, in the corresponding
complexes indicating the coordination of azomethine nitrogen to
the metal ion. Strong n(CQS) bands observed in 1311–1339 cmꢀ1
region for the ligands, which disappeared on complex formation
indicating the coordination of thioenolate sulfur to the [MoO2]2+
Total antioxidant capacity assay. The assay is based on the
reduction of Mo(VI) to Mo(V) by the extract; as was done by
Prieto et al.56 When an acidic pH is maintained subsequent
formation of a green phosphate/Mo(V) complex is seen. The
tubes containing compounds (ligands and complexes 1–3) and
reagent solution (0.6 M sulfuric acid, 28 mM sodium phosphate
and 4 mM ammonium molybdate) were incubated at 95 1C for
90 min. Then, the mixture was cooled to room temperature.
The absorbance was recorded spectrophotometrically for each
solution at 695 nm against a blank.
DPPH radical-scavenging activity. As per the method of
Shimada et al.57 0.004% methanolic solution of DPPH (2,2-
diphenyl-1-picrylhydrazyl) was prepared. Then, various concen-
trations of the compounds (ligands and complexes 1–3) were
added to it. The mixture was shaken vigorously and left to stand
for 30 min in the dark. Gradual fading of a purple colour were
measured at 517 nm against a blank. Ascorbic acid was used as
a positive control. The degree of scavenging was calculated
using the following equation:
centre. Bands at 635–664 cmꢀ1 for the complexes are assigned to
62
n(Mo–N)
.
Medium intensity bands for the complexes at around
430–446 cmꢀ1 are assigned to n(Mo–S)
.
The complexes exhibit
63
two very strong n(MoQO) bands in the region 900–935 cmꢀ1
,
characteristic of the symmetric and antisymmetric stretching
vibrations of the cis-[MoO2]2+ moiety.64,65
1H NMR data for the ligands and the corresponding com-
plexes (1–3) in CDCl3 are summarized in the Experimental
section. The relatively broad signals at d 10.72 and d 10.02 ppm;
d 10.81 and d 10.41 ppm; d 11.28 and d 10.25 ppm in the 1H NMR
spectra corresponding to the O–H and N–H bonds of ligands H2L1,
H2L2 and H2L3 are found to disappear in the complexes 1, 2 and 3,
respectively, as a result of complexation. The signals at d 8.04,
d 8.06 and d 7.91 ppm of the ligands shift downfield to d 9.09,
d 8.85 and d 8.84 ppm indicating coordination from the
azomethine nitrogen in the complexes. The methylene protons
of the ligands and their corresponding complexes 1, 2 and 3
appearing at d 4.59 and d 4.43 ppm; d 4.62 and d 4.47 ppm;
d 4.60 and d 4.42 ppm indicate that the S-benzyl sulfur is not
involved in the coordination. In ligand H2L1, nine aromatic
protons, and in ligands H2L2 and H2L3 eight aromatic protons
appear as multiplets within the ranges d 6.92–7.43 ppm, d 7.08–
7.43 ppm and d 6.88–7.49 ppm, respectively. In complex 1,
Scavenging effect (%) = {(A0 ꢀ A1)/A0} ꢂ 100
Where A0 is the absorbance of the control and A1 is the
absorbance if the sample is present.
Results and discussion
Synthesis
The Schiff base ligands H2L1, H2L2 and H2L3 were prepared 26 and in complexes 2 and 3, 24 aromatic protons appear as
by condensing S-benzyl dithiocarbazate with salicylaldehyde, multiplets within the ranges d 7.22–7.68 ppm, d 7.28–7.73 ppm
5-nitrosalicylaldehyde and 5-bromosalicylaldehyde in ethanol. and d 7.28–7.75 ppm, respectively.
The ligands were satisfactorily characterized by elemental
Considering the IR and 1HNMR data together, it is observed
that both the data support each other in respect to the donor
1
analyses, IR and H NMR.
All the binuclear Mo(VI) complexes (1–3) with general sites of the ligands through which they coordinate to the
formula [(MoO2L)2(4,40-azpy)] were prepared by refluxing the [MoO2]2+ centre.
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Electronic spectra
the second solvent CHCl3 molecule and the 4,40-azpy spacer
(calc. 33.05%). In the third step above 600 1C, a B47.31%
weight loss was observed suggesting the removal of two molecules
of –CH2Ph ligand fragments (calc. 47.28%). Upon further heating
the compound decomposes to an unidentified product.
Electronic spectra of the binuclear dioxomolybdenum(VI) (1–3)
complexes were recorded in dry CH2Cl2 and the spectral data
are listed in the Experimental section. Spectra of all the
binuclear [(MoO2L)2(4,40-azpy)] complexes exhibit several bands
in the 404–294 nm range. All the complexes display the lowest
energy absorption maxima in the 404–353 nm range which are
assigned66 to a S(pp) - Mo(dp) LMCT transition involving the
promotion of an electron from the filled HOMO of the ligand to the
empty LUMO of Mo. Other LMCT bands which may be assigned to
nitrogen to molybdenum and oxygen to molybdenum charge
transfer transitions are observed in the 299–294 nm range.66–69
Complexes 2 and 3 show a weight loss of B8% supported by
an exothermic peak around 210–220 1C which corresponds to
the loss of one guest CHCl3 molecule [calc. 8.7% (2) and 8.31%
(3)]. The second weight losses of B16% (2) and B18.1% (3)
start around 275–330 1C suggesting the loss of the second guest
CHCl3 molecule [calc. 17.45% (2) and 16.62% (3)]. Finally, the
major weight loss of B38.1% (2) and B45.9% (3) occurring
until B600 1C corresponds to the loss of the 4,40-azpy spacer
along with two molecules of –CH2Ph ligand fragments [calc.
40.17% (2) and 42.09% (3)]. This suggests retention of the
framework structures of 2 and 3 up to B600 1C indicating
the robustness of the framework even after the removal of the
guest CHCl3 molecules.
Electrochemical properties
Cyclic voltammograms of the binuclear [(MoO2L)2(4,40-azpy)]
complexes (1–3) at a platinum electrode were recorded in dry
degassed DMF containing 0.1 M TBAP as the supporting
electrolyte over the potential range 0.0 to ꢀ1.5 V. The cyclic
voltammetric results of all the binuclear dioxomolybdenum(VI)
complexes are given in Table 1 and the cyclic voltammograms are
shown in Fig. S1 (ESI†). All the complexes show only one irreversible
two-electron one-step reduction within the potential range ꢀ0.92
to ꢀ1.01 V vs. the SCE in DMF solution which corresponds to a
MoVI–MoVI/MoIV–MoIV couple.15,70 Two-electron involvement is
established by a comparison of the current height with authentic
two electron species under identical experimental conditions.
On scan reversal, one irreversible two-electron one step oxidative
response is located in the range 0.44 to 0.47 V which corresponds
to a MoIV–MoIV/MoVI–MoVI couple. It should be noted that
the reduction of binuclear [(MoO2L)2(4,40-azpy)] complexes in
aprotic solvents is generally irreversible.
Thus, the thermogravimetric study reveals that with minor
variations between the curves, all the complexes show similar
decomposition profiles because of the similarities in the metal
coordination modes in the structures.
Description of the crystal structures of complexes 1, 2 and 3
The molecular structures and the atom numbering schemes of
complexes 1, 2 and 3 are shown in Fig. 1–3. Crystallographic
data, relevant bond lengths and bond angles together with
hydrogen bonding geometries are listed in Tables 2–4.
The orange-red isostructural binuclear complexes 1, 2 and 3
contain crystallographic centers of inversion at the middle of
the central NQN bond of the 4,40-azpy molecule. In the crystal
structures a solvent chloroform molecule is found in the
asymmetric unit but in all three cases, the molecule is disordered
with two overlapping molecules given population parameters x and
1 ꢀ x with x refined to values close to 0.5. In all the complexes each
Mo(1) is bonded to two oxo atoms O(2) and O(3), phenolate
oxygen O(1), thioenolate sulfur S(1), azomethine nitrogen N(1)
Thermogravimetric analysis and framework stability
Thermogravimetric analyses of complexes 1–3 were conducted
in the temperature range 30–700 1C with a 10 1C minꢀ1 interval
in nitrogen atmosphere to assess their framework stabilities
(Fig. S2–S4, ESI†). Above 200 1C, the TG/DTA curve exhibits
three steps of weight loss. Complex 1 shows a weight loss of
B8% supported by an exothermic peak around 210–220 1C,
which corresponds to the loss of one guest solvent CHCl3
molecule (calc. 9.3%) and the framework is quite stable up to
275 1C. In the temperature range 275–330 1C, a B30.19%
weight loss was observed suggesting the concomitant loss of
Table 1 Cyclic voltammetric resultsa (V vs. SCE) for the [(MoVIO2L)2(4,40-
azpy)] complexes at 298 K
Mo(VI)–Mo(VI)/
Mo(IV)–Mo(IV)
Epc (V)
Mo(VI)–Mo(VI)/
Mo(IV)–Mo(IV)
Epa (V)
Complexes
[(MoO2L1)2(4,40-azpy)] (1)
[(MoO2L2)2(4,40-azpy)] (2)
[(MoO2L3)2(4,40-azpy)] (3)
ꢀ0.92
ꢀ0.98
ꢀ1.01
+0.47
+0.44
+0.45
a
Fig. 1 View of the coordination environment around hexa-coordinated
Mo(VI) with an atom labeling scheme in the centrosymmetric complex 1,
the disordered solvent molecule is omitted for clarity.
Solvent: DMF (dry, degassed); supporting electrolyte: 0.1 M TBAP;
solution strength: 10ꢀ3 M; working electrode: platinum; reference
electrode: SCE; scan rate: 200 mV sꢀ1
.
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pyridine nitrogens and the two azo nitrogens. The molybdenum
centre preferably coordinates to the two pyridine nitrogens since
they are more readily accessible and also sterically favorable for the
formation of binuclear complexes. The N-donor auxiliary ligand
(4,40-azpy) in 1–3 acts as a bis-monodentate ligand occupying one
axial position and connecting two molybdenum(VI) atoms which are
13.816(2), 13.822(1) and 13.948(1) Å apart, respectively. No doubt
because of the trans influence of O(3), the Mo(1)–N(3) bond lengths
[2.435(8), 2.423(4) and 2.456(5) Å] are considerably longer than the
other Mo(1)–N(1) bonds [2.259(8), 2.285(4) and 2.276(5) Å], values
which indicate that the azpy nitrogen is comparatively weakly
bonded to the [MoO2]2+ centre pointing to the possibility of ligand
exchange reaction at this point. As the ligand is coordinated in
the thiolate form, the C(8)–S(1) bond is expected to assume a
single bond character but bond lengths are 1.709(10), 1.748(5)
and 1.746(6) Å, respectively, distances which lie between the
expected lengths of C–S and CQS bonds and may be attributed
to electron delocalization in the coordinated ligands.72,73 The
adjacent C(8)–N(2) bond lengths are 1.289(11), 1.293(6) and
1.284(8) Å and are closer to the CQN distance than the normal
C–N distance. The N(1)–N(2) bond distances 1.398(10), 1.409(5)
and 1.410(7) Å, respectively, reveal their predominant single
bond character.
Fig. 2 View of the coordination environment around hexa-coordinated
Mo(VI) with an atom labeling scheme in the centrosymmetric complex 2,
the disordered solvent molecule is omitted for clarity.
Since the auxiliary ligand and the [MoO2L] layer are almost
perpendicular to each other, interestingly considerable solvent-
accessible voids between the layers are formed resulting in the
formation of rhombus-shaped cavities. The cavities having
dimensions 13.816 Å ꢂ 10.827 Å (1), 13.822 Å ꢂ 10.925 Å (2)
and 13.948 Å ꢂ 11.180 Å (3) which are occupied by CHCl3
molecules are further confirmed by TG and elemental analyses.
For a better understanding of the supramolecular framework
formation, the various interactions present within the crystals are
pictorially depicted in Fig. 4–6 for complexes 1, 2 and 3, respectively.
In complex 1, the molybdenum oxo-oxygen (trans to 4,40-azpy)
and the coordinated sulphur behave as hydrogen bond acceptors.
Adjacent binuclear complexes are linked via H(14)ꢁꢁꢁO(3) and
H(13)ꢁꢁꢁS(1) hydrogen bonding interactions resulting in a chain
Fig. 3 View of the coordination environment around hexa-coordinated
Mo(VI) with an atom labeling scheme in the centrosymmetric complex 3,
the disordered solvent molecule is omitted for clarity.
of the ligand and nitrogen atom N(3) of the bridging 4,40-azpy type 2D supramolecular framework which propagates along the
molecule in a distorted octahedral coordination environment a-axis (Fig. S5, ESI†). Parallel chains further self-assemble via
around the metal centre. The ligands are active in their enolate H(2)ꢁ ꢁ ꢁO(2) hydrogen bonding interactions. The oxo-oxygen (cis
forms and behave in a dianionic tridentate manner. Three to 4,40-azpy) participates in hydrogen bonding interaction with
donor atoms O(1), N(1), and S(1) of the ligand and one terminal the hydrogen of the salicylaldehyde ring finally leading to the
oxygen O(2) occupy the equatorial plane.67 The dianionic construction of a 3D supramolecular pillared-layer framework
tridentate ligands form one five-membered and another six- (Fig. 7). The oxo-oxygens of the [MoO2]2+ moiety not only form
membered chelating ring around the [MoO2]2+ centre with bite hydrogen bonds to extend the chain structure but also behave as
angles of N(1)–Mo(1)–S(1), 75.8(2)1, 76.16(1)1 and 75.98(1)1 and hydrogen bond acceptors to anchor the guest CHCl3 molecules
N(1)–Mo(1)–N(3), 81.7(3)1, 81.87(1)1 and 82.01(2)1, respectively. to the host framework via H(21)ꢁ ꢁ ꢁO(3) and H(21)ꢁ ꢁ ꢁO(2) inter-
The oxo-oxygen O(3) occupies an axial position in the coordina- actions. Fig. S6 (ESI†) also portrays the staircase architecture of
tion sphere of Mo(1). The other axial position is occupied by the complex 1 along the b-axis.
nitrogen atom N(3) of one half of the 4,40-azpy unit. In com-
Complex 2 is rich in hydrogen bonding interactions. The
plexes 1, 2 and 3, the Mo(VI) centre are displaced by 0.257, 0.268 molybdenum oxo-oxygens are involved in hydrogen bonding
and 0.328 Å, respectively, from the equatorial plane in the interactions with hydrogen atoms in the phenyl ring of the
direction of the terminal oxygen O(3). The coordination mode S-benzyl moiety (Fig. S7, ESI†). Similar to complex 1, here also
of the Schiff base ligands allows the inclusion of the auxiliary the adjacent binuclear complexes are linked via H(13)ꢁ ꢁ ꢁO(2)
ligand 4,40-azpy which is an organic ‘‘distorted’’ rung71 unit. The and H(12)ꢁ ꢁ ꢁO(3) hydrogen bonding interactions resulting
4,40-azpy ligand has four possible coordination sites, namely the two in an extended chain architecture along the b-axis (Fig. 8).
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Table 2 Crystal data and details of refinement for complexes 1–3
Complex 1
Complex 2
Complex 3
Chemical formula
Formula weight (M)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
a (1)
b (1)
C42H34N8S4O6Cl6Mo2
C42H32N10S4O10Cl6Mo2
1369.60
Triclinic
%
P1
10.926(7)
10.964(7)
13.235(8)
67.269(6)
69.255(5)
67.937(6)
C42H32N8 S4O6Cl6Br2Mo2
1437.38
Triclinic
%
P1
11.166(5)
11.179(5)
13.139(6)
67.863(10)
65.724(10)
67.080(10)
1279.59
Triclinic
%
P1
10.827(18)
11.070(18)
12.580(2)
65.780(17)
73.533(17)
65.932(17)
g (1)
V (Å3)
Z
1243(4)
1
1314(16)
1
1330.51(10)
1
Crystal size
0.18 ꢂ 0.16 ꢂ 0.14
0.24 ꢂ 0.03 ꢂ 0.03
0.20 ꢂ 0.18 ꢂ 0.16
Temperature (K)
296(2)
150(2)
296(2)
DC (g cmꢀ3
)
1.709
1.050
640
0.943
1.731
1.006
684
1.039
1.794
2.484
708
0.882
m(Mo Ka) (mmꢀ1
F(000)
)
Goodness of fit on F2
No of independent reflections
No of reflections with I 4 2s(I)
R1, wR2 [I 4 2s(I)]
3437
2429
7678
5698
R1 = 0.0590
wR2 = 0.1243
2.214, ꢀ2.235
4959
3683
R1 = 0.0416
wR2 = 0.1037
1.072, ꢀ0.660
R1 = 0.0838
wR2 = 0.2067
1.204, ꢀ1.733
P
Max, min residual electron density e Åꢀ3
P
P
P
R = ||F0| ꢀ |Fc||/ |F0|; wR(F2) = [ w(|F0|2 ꢀ |Fc|2)2/ w|F0|4]1/2
.
Table Geometry of hydrogen-bonding interactions in the crystal additional H(13)ꢁ ꢁ ꢁS(1) hydrogen bonding interactions involving
3
structures of complexes 1–3a
the coordinated sulphur and hydrogen atoms of the phenyl ring of
the S-benzyl moiety which assist in the formation of a 3D robust
framework.
D–H
A
Hꢁ ꢁ ꢁA (Å) Dꢁ ꢁ ꢁA (Å) D–Hꢁ ꢁ ꢁA (1) Symmetry code
Complex 1
C(14)–H(14)
C(13)–H(13)
C(21A)–
Similarly in complex 3, the oxo-oxygens actively participate
in hydrogen bonding. The rhombus grid framework is basically
formed by H(13)ꢁ ꢁ ꢁO(2) and H(12)ꢁ ꢁ ꢁO(3) hydrogen bonding
interactions which help to extend the chain along the b-axis
(Fig. S8, ESI†). The chains extend in three-dimensions by
H(3)ꢁ ꢁ ꢁO(2) hydrogen bonding interactions forming a fascinating
supramolecular pillared layer structure (Fig. 9). The guest CHCl3
molecules are held in the cavities (Fig. 6) by strong H(21)ꢁ ꢁ ꢁO(2)
hydrogen bonding interactions.
There are no pꢁ ꢁ ꢁp interactions in complexes 1–3 since the
shortest centroid–centroid distances between adjacent aromatic
rings are all 410 Å, much larger than those expected between
the pyridyl rings for this type of interaction20 [10.827 Å (1), 10.925 Å
(2), and 11.180 Å (3)].
O(3) 2.92
S(1) 3.01
O(3) 2.72
3.78(2) 154.8
3.77(1) 140.7
3.47(2) 133.3
1 + x, y, z
1 + x, y, z
x, y, z
H(21A)
C(21A)–
H(21A)
C(21B)–
H(21B)
C(2)–H(2)
O(2) 2.38
O(2) 2.62
O(2) 2.69
3.16(2) 136.3
3.40(2) 136.3
3.90(2) 125.5
x, y, z
x, y, z
1 ꢀ x, ꢀy, 1 ꢀ z
Complex 2
C(13)–H(13)
C(12)–H(12)
C(19)–H(19)
C(13)–H(13)
C(21A)–
O(2) 2.88
O(3) 3.01
O(5) 2.47
S(1) 3.10
O(3) 3.01
3.52(2) 126.4
3.91(2) 162.3
3.39(2) 170.4
3.82(1) 136.0
3.72(2) 130.3
1 + x, y, z
1 + x, y, z
x, y, ꢀ1 + z
1 + x, y, z
1 ꢀ x, 1 ꢀ y, 1 ꢀ z
H(21A)
Complex 3
C(21A)–
H(21A)
O(2) 2.22
3.33(2) 146.0
1 ꢀ x, ꢀy, 2 ꢀ z
SEM investigations
The morphology and structure of the three binuclear dioxomolyb-
denum(VI) complexes 1–3 were examined using a scanning electron
microscope. The samples were spread over a small piece of carbon
tape and then coated with a 5 nm (approx.) thickness of Au using a
BIO-RAD POLARAN sputter coater. The SEM images of the com-
plexes are shown in Fig. 10. The images reveal that the compounds
form crystalline structures in the form of microrods and microplates
with dimensions of B2 mm. They have smooth surfaces and sharp
edges and exhibit some degree of aggregation.
C(13)–H(13)
C(12)–H(12)
C(3)–H(3)
O(2) 2.82
O(3) 3.12
O(2) 2.83
3.49(2) 129.9
4.01(1) 160.7
3.41(2) 121.7
x, 1 + y, z
x, y, z
1 ꢀ x, ꢀy, 2 ꢀ z
a
C(21)–H(21) is a part of the chloroform molecule.
Parallel chains self assemble via H(19)ꢁ ꢁ ꢁO(5) hydrogen bond-
ing interactions. The oxygen atom of the nitro group helps to
extend the chain to a 3D supramolecular framework with
rhombus cavities (Fig. 8). The guest CHCl3 molecules are entrapped
in the rhombus cavities by H(21A)ꢁꢁꢁO(3) hydrogen bonding inter-
DFT and TD-DFT calculations of complexes 1, 2 and 3
actions. In addition to the above described hydrogen bonding The full geometry optimizations of 1, 2 and 3 have been carried out
interactions that lead to framework formation, there are using the DFT/B3LYP methodology. The calculated dimensions are
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Table 4 X-ray and calculated (DFT/B3LYP method) bond distances (Å) and angles (deg) in 1–3
Complex 1
Complex 2
Complex 3
Obs.
Calc.
1.980
1.719
1.707
2.333
2.544
2.468
1.760
1.295
1.383
Obs.
Calc.
1.996
1.717
1.706
2.340
2.528
2.457
1.763
1.296
1.382
Obs.
Calc.
Bond distances (Å)
Mo(1)–O(1)
Mo(1)–O(2)
Mo(1)–O(3)
Mo(1)–N(1)
Mo(1)–N(3)
Mo(1)–S(1)
C(8)–S(1)
1.920(6)
1.692(6)
1.670(7)
2.259(9)
2.435(8)
2.416(4)
1.709(10)
1.289(11)
1.398(10)
1.961(3)
1.713(3)
1.700(3)
2.285(4)
2.423(4)
2.428(1)
1.748(5)
1.293(6)
1.409(5)
1.929(3)
1.703(3)
1.693(4)
2.285(4)
2.458(4)
2.435(2)
1.745(5)
1.282(6)
1.400(5)
1.983
1.718
1.707
2.339
2.538
2.465
1.761
1.296
1.380
C(8)–N(2)
N(1)–N(2)
Bond angles (deg)
O(2)–Mo(1)–O(1)
O(3)–Mo(1)–O(1)
O(3)–Mo(1)–O(2)
O(1)–Mo(1)–N(1)
O(2)–Mo(1)–N(1)
O(3)–Mo(1)–N(1)
O(1)–Mo(1)–S(1)
O(2)–Mo(1)–S(1)
O(3)–Mo(1)–S(1)
N(1)–Mo(1)–S(1)
O(1)–Mo(1)–N(3)
O(2)–Mo(1)–N(3)
O(3)–Mo(1)–N(3)
N(1)–Mo(1)–N(3)
S(1)–Mo(1)–N(3)
C(7)–N(1)–Mo(1)
N(2)–N(1)–Mo(1)
C(16)–N(3)–Mo(1)
C(20)–N(3)–Mo(1)
C(1)–O(1)–Mo(1)
C(8)–S(1)–Mo(1)
O(1)–C(1)–C(6)
106.0(3)
99.5(3)
105.5(3)
81.7(3)
160.2(3)
90.9(3)
150.6(2)
90.3(3)
99.5(3)
75.8(2)
76.8(3)
82.4(3)
106.1
105.3(2)
98.3(2)
106.2(2)
81.8(1)
160.9(1)
89.8(2)
151.4(1)
90.8(1)
99.7(1)
76.1(1)
77.1(1)
83.0(2)
106.2
104.9(2)
99.6(2)
105.8(2)
82.1(1)
160.6(2)
90.4(2)
151.5(1)
90.9(1)
98.4(2)
75.9(1)
77.5(1)
82.5(2)
106.2
99.3
106.5
80.1
159.1
91.6
147.9
91.0
101.4
75.0
76.0
80.8
172.3
81.5
99.5
106.6
80.1
158.6
92.1
148
90.8
101.2
75.0
75.8
80.7
172.3
81.1
98.6
106.5
80.0
159.6
91.4
148.1
91.3
101.6
75.1
75.9
81.1
171.8
81.6
80.9
122.8
122.6
124.0
117.7
131.9
100.8
122.1
123.3
126.2
113.6
114.6
126.6
172.0(3)
81.7(3)
81.4(2)
170.6(2)
81.4(1)
81.6(1)
171.6(2)
81.4(1)
81.4(1)
80.6
80.4
124.6(7)
123.1(5)
126.6(5)
117.9(6)
135.1(6)
100.3(3)
122.2(8)
124.1(8)
124.4(9)
111.4(7)
112.1(7)
128.3(8)
122.6
122.9
123.7
117.9
131.4
100.7
122.0
123.0
126.3
113.6
114.6
126.5
124.6(3)
122.5(3)
124.1(3)
118.8(3)
135.9(3)
100.1(2)
121.9(4)
123.7(4)
125.9(4)
112.2(4)
113.0(4)
127.5(4)
124.(3)
122.8
122.8
117.5
124.2
131.6
100.7
122.1
123.2
126.2
113.6
114.6
126.5
122.5(3)
118.8(3)
124.3(3)
136.8(3)
100.0(2)
121.4(4)
124.1(4)
125.5(4)
112.4(4)
113.6(4)
127.4(4)
C(1)–C(6)–C(7)
N(1)–C(7)–C(6)
C(7)–N(1)–N(2)
C(8)–N(2)–N(1)
N(2)–C(8)–S(1)
given in Table 4 and are in good agreement with the experimental Again weak transitions at 440–470 nm have been observed
data. The maximum deviation in the bond length between experi- for p(L) - p*(L)/dp(Mo) transitions for 1 and 2, while a mixed
mental and calculated is 0.08–0.11 Å for Mo(1)–N(3).
p(L) - p*(L)/dp(Mo) and p(L) - p*(L)/p*(oxo) character in
The compositions along with the energies of some selected complex 3. The moderately intense transitions at B400 nm
molecular orbitals of complexes are given in Tables S1–S3 have a p(L) - p*(azpy) character which has been experi-
(ESI†). Contour plots of some selected frontier molecular mentally observed for the complexes in dichloromethane. The
orbitals of the complexes are shown in Fig. 11–13. The higher high energy transitions at B300 nm again have p(L) - p*(azpy)
energy occupied molecular orbitals (HOMO to HOMOꢀ9) have in the complexes.
significant contributions from ligand L, while HOMOꢀ10 is
Antimicrobial activity
concentrated on bridging the azpy moiety in the complexes.
The lowest energy unoccupied molecular orbital (LUMO) has a The Schiff base ligands and all the synthesized molybdenum(VI)
p*(azpy) character. Other lowest energy unoccupied molecular complexes were tested against two gram-positive (Bacillus cerus,
orbitals (LUMO+1 to LUMO+8) have a mixed dp(Mo) and p*(L) Listeria monocytogenes) and two gram-negative (Escherichia coli,
character along with a significant contribution from pp(O) Staphylococcus aureus) bacteria by the disc diffusion method.
orbitals.
All the compounds exhibited different degrees of antimicrobial
To obtain further insight into the electronic spectra of the activity at a concentration of 10 mg per disc against the test
complexes, vertical electronic excitations have been calculated pathogens. The qualitative and quantitative antimicrobial test
by the TDDFT/CPCM method in dichloromethane solvent. The results are presented in Tables 8 and 9 respectively. The anti-
calculated excitation wavelengths and their assignments are microbial activities were comparable with those of commonly
summarized in Tables 5–7. In the complexes very weak transi- used antibiotics (ampicillin and tetracycline) (reference disk)
tions at 486–517 nm correspond to p(L) - p*(azpy) transitions. against the test pathogens. Among all the screened compounds,
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Fig. 6 Packing diagram of complex 3 showing the hydrogen bonding
interactions H(21)ꢁ ꢁ ꢁO(2) (blue dotted line), H(13)ꢁ ꢁ ꢁO(2) (red dotted line),
H(12)ꢁ ꢁ ꢁO(3) (purple dotted line) and H(3)ꢁ ꢁ ꢁO(2) (green dotted line), non-
bonding hydrogen atoms are omitted for clarity.
Fig. 4 Packing diagram of complex 1 showing the intermolecular hydrogen
bonding interactions H(14)ꢁꢁꢁO(3) (green dotted line), H(13)ꢁꢁꢁS(1) (orange
dotted line), H(21)ꢁꢁꢁO(3) (red dotted line), H(21)ꢁꢁꢁO(2) (blue dotted line) and
H(2)ꢁꢁꢁO(2) (magenta dotted line), non-bonding hydrogen atoms are omitted
for clarity.
Fig. 7 Spacefilling representation illustrating the formation of a supra-
molecular pillared-layer structure in complex 1, the azpy pillars are shown
in blue and the MoO2L1 layer is shown in pink, hydrogen atoms and solvent
molecules are omitted for clarity.
Fig. 5 Packing diagram of complex 2 along the b-axis showing the
hydrogen bonding interactions H(13)ꢁ ꢁ ꢁO(2) (blue dotted line), H(12)ꢁ ꢁ ꢁO(3)
(green dotted line), H(19)ꢁ ꢁ ꢁO(5) (purple dotted line) H(13)ꢁ ꢁ ꢁS(1) (black
dotted line) and H(21A)ꢁ ꢁ ꢁO(3) (red dotted line), non-bonding hydrogen
atoms are omitted for clarity.
the Schiff base ligands (H2L1–H2L3) showed moderate to good
antimicrobial activity against all these test microorganisms.
Fig. 8 Spacefilling representation illustrating the formation of a supra-
molecular pillared-layer structure in complex 2, the azpy pillars are shown
in blue and the MoO2L2 layer is shown in orange, hydrogen atoms and
solvent molecules are omitted for clarity.
Complex
1 showed moderate to good activity against
Escherichia coli and Listeria monocytogenes and exhibited weak
activity against other test organisms. Complex 2 showed good
activity against the tested pathogens and complex 3 also
showed moderate activities against the pathogens. From
Table 8 it is observed that ligands H2L1–H2L3 have better
antimicrobial activity than their corresponding complexes.
The MIC of the ligands and their corresponding molyb-
The Schiff base ligands were also found to have comparable denum complexes against the test microorganisms was further
activity to the two established drugs against the bacterial determined using sterilized discs saturated with different con-
strains studied under the test conditions, showing that they centrations (0.25–10 mg per disc) of test compounds. The MIC
have a good activity as bactericides.
value of the compounds for antimicrobial activity ranges
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Fig. 9 Spacefilling representation illustrating the formation of a supra-
molecular pillared-layer structure in complex 3, the azpy pillars are shown
in blue and the MoO2L3 layer is shown in green, hydrogen atoms and
solvent molecules are omitted for clarity.
Fig. 12 Contour plots of selected molecular orbitals of complex 2.
Fig. 10 SEM images of complexes 1, 2 and 3.
Fig. 13 Contour plots of selected molecular orbitals of complex 3.
Antioxidant activity
The model of the scavenging of a stable DPPH (2,2-diphenyl-1-
picrylhydrazyl) radical is commonly employed for the assay of
antioxidant activity of compounds across a short time scale.
The use of the stable DPPH radical (due to extensive deloca-
lization of the unpaired electron) has the advantage of being
less affected by side reactions such as enzyme inhibition and
metal chelation.77 DPPH accepts an electron or hydrogen to
become a stable diamagnetic radical. Due to its unpaired
electron, DPPH gives a strong absorption band at 517 nm
giving a deep violet colour.78 As the electron becomes paired
up in the presence of the Schiff base ligands (H2L1–H2L2) and
complexes 1–3, the absorption diminishes giving a yellow
colour and this resulting decolourization is stoichiometric
with respect to the number of electrons taken up. With the
increase in concentration of the test compounds a marked
decrease in absorption was observed indicating strong anti-
oxidant properties of the test compounds.
Fig. 11 Contour plots of selected molecular orbitals of complex 1.
between 0.5 and 10 mg per disc (Table 9). The results reveal that
the Schiff base ligands74–76 and the synthesized complexes 1–3
may be proposed as potential new candidates in antimicrobial
treatment. Thus, from the MIC and average inhibition zone
results, it is revealed that H2L1 and H2L2 posses the highest
activity among all the compounds against the test microorgan-
isms. A graphical representation of the inhibition zones of the
compounds is given in Fig. 14.
The total antioxidant capacity (TAC) of the compounds in
terms of the ascorbic acid equivalent (AAE) is given in Table 10
and DPPH radical scavenging activity is given in Table 11.
Ligand H2L2 gave the best results for total antioxidant capacity
assay as 1 mg of sample was equivalent to 0.09 mg of ascorbic
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Table 5 Calculated vertical electronic excitations in complex 1
Eexcitation (eV)
lexcitation (nm)
Osc. strength ( f )
Key transitions
Character
2.4002
2.6628
516.6
465.6
0.0140
0.0471
(92%) HOMO - LUMO
p(L1) - p*(azpy)
(49%) HOMOꢀ1 - LUMO+1
(41%) HOMO - LUMO+2
(74%) HOMOꢀ4 - LUMO
(86%) HOMOꢀ8 - LUMO
(33%) HOMO - LUMO+6
(27%) HOMOꢀ1 - LUMO+5
(67%) HOMOꢀ11 - LUMO
(65%) HOMOꢀ12 - LUMO
p(L1) - p*(L1)/dp(Mo)
2.9688
3.2172
3.5175
417.6
385.4
352.5
0.0719
0.0423
0.4260
p(L1) - p*(azpy)
p(L1) - p*(azpy)
p(L1) - p*(L1)/dp(Mo)
3.7006
4.0353
335.0
307.3
0.0513
0.2163
p(L1) - p*(azpy)
p(L1) - p*(azpy)
Table 6 Calculated vertical electronic excitations in complex 2
Eexcitation (eV)
lexcitation (nm)
Osc. strength ( f )
Key transitions
Character
2.5520
2.7652
485.8
448.4
0.0202
0.0185
(78%) HOMOꢀ1 - LUMO
(34%) HOMOꢀ1 - LUMO+1
(32%) HOMOꢀ1 - LUMO+2
(34%) HOMO - LUMO+1
(32%) HOMO - LUMO+2
(81%) HOMOꢀ2 - LUMO
(73%) HOMOꢀ6 - LUMO
(48%) HOMOꢀ9 - LUMO
(39%) HOMO - LUMO+6
(25%) HOMOꢀ1 - LUMO+5
(65%) HOMOꢀ11 - LUMO
p(L2) - p*(azpy)
p(L2) - p*(L2)/dp(Mo)
2.7671
448.1
0.0266
p(L2) - p*(L2)/dp(Mo)
2.8680
3.1720
3.3352
3.7309
432.3
390.9
371.75
332.3
0.0240
0.1293
0.0457
0.0845
p(L2) - p*(azpy)
p(L2) - p*(azpy)
p(L2) - p*(azpy)
p(L2) - p*(L2)/dp(Mo)
4.1159
301.2
0.0792
p(L2) - p*(azpy)
Table 7 Calculated vertical electronic excitations in complex 3
Eexcitation (eV)
lexcitation (nm)
Osc. strength ( f )
Key transitions
Character
2.4088
2.6436
514.72
468.99
0.0102
0.0234
(85%) HOMO - LUMO
p(L3) - p*(azpy)
(45%) HOMOꢀ1 - LUMO+1
(40%) HOMO - LUMO+2
(43%) HOMOꢀ1 - LUMO+1
(41%) HOMOꢀ1 - LUMO+3
(90%) HOMOꢀ4 - LUMO
(69%) HOMOꢀ8 - LUMO
(34%) HOMOꢀ5 - LUMO+3
(32%) HOMOꢀ3 - LUMO+4
(35%) HOMOꢀ1 - LUMO+5
(29%) HOMO - LUMO+6
(72%) HOMOꢀ12 - LUMO
p(L3) - p*(L3)/dp(Mo)
p(L3) - p*(L3)/p*(oxo)
p(L3) - p*(L3)/dp(Mo)
p(L3) - dp(Mo)/ p*(oxo)
p(L3) - p*(azpy)
2.6499
467.88
0.0251
2.9596
3.2152
3.2469
3.3839
3.6550
418.92
385.62
381.86
366.40
339.2
0.0491
0.0752
0.0208
0.0344
0.0692
p(L3) - p*(azpy)
p(L3) - dp(Mo)/p*(oxo)
p(L3) - dp(Mo)/p*(oxo)
p(L3) - p*(L3)/dp(Mo)
4.1065
301.9
0.0812
p(L3) - p*(azpy)
Table 8 Antimicrobial activity of the ligands and complexes 1–3
Table 9 Minimum inhibitory concentration (MIC) (mg per disc) of the
ligands and complexes 1–3
Test pathogens (average inhibition zone in mm)
Test pathogens
Compounds (10 Escherichia Bacillus Listeria
Staphylococcus
subtilis monocytogenes aureus
mg per disc)
coli
Escherichia Bacillus
Listeria
monocytogenes aureus
Staphylococcus
Compounds coli
subtilis
H2L1
19.5
14
17
16
15
10
14
—
12
7
18
12
20
16
11
10
30
18
9
8
8
—
10
8.5
H2L1
1
0.5
0.5
0.5
0.5
0.5
1
0.5
5
0.5
1
0.5
0.5
0.5
1
0.5
0.5
0.5
0.5
1
1
1
10
1
1
1
H2L2
14
13
13
11
—
—
H2L2
2
2
H2L3
H2L3
3
3
Ampicillin
Tetracycline
—
24
[—] = no inhibition; [NT] = not tested.
Conclusions
acid. Moreover, H2L2 had 76.56% DPPH radical scavenging
We have successfully synthesized three new isostructural binuc-
lear dioxomolybdenum(VI) complexes by using Schiff base
ligands together with 4,40-azopyridine as the spacer. All three
capacity which is the highest among the six compounds ana-
lyzed at a low concentration of 50 mg mlꢀ1
.
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Fig. 14 Zone of inhibition (mm) of the compounds(10 mg per disc) for the pathogens.
Table 10 TAC values of the ligands and complexes 1–3
diffractometer, Dr Ganesh C. Sahoo (CSIR-CGCRI) for the
thermal analysis and Dr Pramit Chowdhury (IIT Delhi) for the
SEM study. S. Biswas acknowledges UGC, New Delhi, and T. K.
Mondal acknowledge DST, New Delhi.
H2L1
1
H2L2
2
H2L3
3
TAC (AAE) (mg mgꢀ1
)
0.08
0.06
0.09
0.06
0.09
0.06
Table 11 DPPH radical scavenging activity of the ligands and complexes
1–3
References
ˇ
H2L1
1
H2L2
2
H2L3
3
´
´
´
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