Synthesis and characterization of Mo(VI) complexes derived from ONO donor acylhydrazones

Article abstract of DOI:10.1016/j.saa.2011.01.021

Four new dioxomolybdenum(VI) complexes were prepared using different acylhydrazones. Hydrazones used for complexation were derived from 2-hydroxy-4-methoxybenzaldehyde, 2-hydroxy-4-methoxyacetophenone, benzhydrazide and nicotinoyl hydrazide. The complexes

Full text of DOI:10.1016/j.saa.2011.01.021

Spectrochimica Acta Part A 78 (2011) 1424–1428
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis and characterization of Mo(VI) complexes derived from ONO donor
acylhydrazones
Nancy Mathew, M.R. Prathapachandra Kurup∗
Department of Applied Chemistry, Cochin University of Science and Technology, Kochi 682022, Kerala, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Four new dioxomolybdenum(VI) complexes were prepared using different acylhydrazones. Hydra-
zones used for complexation were derived from 2-hydroxy-4-methoxybenzaldehyde, 2-hydroxy-4-
methoxyacetophenone, benzhydrazide and nicotinoyl hydrazide. The complexes were characterized by
various spectroscopic techniques, thermal and elemental analyses. The 1H NMR and IR spectral data
indicate tridentate nature of the ligands through enolization. IR spectra provide information about the
dimeric nature of the complexes. The thermal analyses of the complexes showed the presence of lattice
water in some of the complexes.
Received 28 September 2010
Received in revised form
31 December 2010
Accepted 17 January 2011
Keywords:
Acylhydrazones
Dioxomolybdenum complexes
Spectral studies
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
of dioxomolybdenum complexes with O, N and S containing ligands
[10–12]. Ligands with heteroatom in their structure act as good
Recently molybdenum chemistry has gained considerable
attention due to versatile applications of molybdenum and its
complexes in the fields of catalysis and biology [1,2]. Among
the second series of transition metals, molybdenum is the only
biometal important for living kingdom with a large number of
stable and accessible oxidation states. The importance of molyb-
denum as a biological trace element depends on its contribution in
various molybdoenzymes. The dioxomolybdenum complexes have
been widely studied, as models for the active sites of oxotransfer
molybdoenzymes like sulfite and aldehyde oxidase, xanthine oxi-
dase, xanthine dehydrogenase and nitrate reductase [3–5]. In the
catalytic activity of molybdoenzymes, the oxidation state of molyb-
denum varies between VI and IV states and Mo(V) coexists with
chelating agents for complex formation. The intensified interests in
the study of metal complexes with hydrazones are consequences
of their increasing applications in different areas like medicinal,
biological, industrial and analytical chemistry [13–15]. Our inter-
est behind the study of molybdenum complexes with hydrazones
lies in the fact that the information gained during the study of these
complexes may be useful to reveal the enzyme structures and their
functions which are not readily obtained by studying the enzymes
themselves. As an extension of our work on the syntheses and
characterization of transition metal complexes with hydrazones
[16,17], here we report the syntheses and spectral perspectives
of four dioxomolybdenum complexes with different ONO donor
hydrazones.
2+
Mo(VI) and Mo(IV) [6]. The presence of cis-MoO2 group in the
oxidized form of some molybdoenzymes stimulate the search for
new compounds in which this moiety is coordinated to ligands con-
taining heteroatoms like O, N and S. The cofactors of these enzymes
are most probably coordinatively unsaturated and this helps the
easy binding of substrate. The study of molybdenum complexes
with dianionic tridentate ligands is particularly significant because
2. Experimental
2.1. Materials
2-Hydroxy-4-methoxyacetophenone (Aldrich), 2-hydroxy-
4-methoxybenzaldehyde (Aldrich), nicotinic acid hydrazide
(Aldrich), benzhydrazide (Aldrich) and MoO2(acac)2 (Aldrich) were
used without further purification. Solvent used was methanol.
2+
the coordination of cis-MoO2 with dianionic tridentate ligand
systems provides an open active site on molybdenum [7]. The cat-
alytic activities of dioxomolybdenum complexes were proved by
several reports [8,9]. There are many reports regarding the studies
2.2. Synthesis
2.2.1. Syntheses of acylhydrazones
∗
The syntheses of 2-hydroxy-4-methoxybenzaldehyde nicoti-
noylhydrazone monohydrate (H2hmbn·H2O) and 2-hydroxy-
Corresponding author. Tel.: +91 484 2862423; fax: +91 484 2575804.
E-mail address: mrp k@yahoo.com (M.R.P. Kurup).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.01.021

N. Mathew, M.R.P. Kurup / Spectrochimica Acta Part A 78 (2011) 1424–1428
1425
O
OH
H2N
O
O
OH
R
H3C
O
H3C
reflux 4 h
-H O
NH
+
N
O
NH
solvent-methanol
R
X
X
R = H & X=N; H hmbn
R = CH & X= N; H hman
R = H & X=C; H2hmbb
R = CH & X=C; H hmab
Scheme 1. Syntheses of hydrazones.
4-methoxyacetophenone
have been reported by us earlier [16,17]. 2-Hydroxy-4-
methoxybenzaldehyde benzoylhydrazone (H2hmbb) and
2-hydroxy-4-methoxyacetophenone benzoylhydrazone (H2hmab)
were synthesized by refluxing an equimolar mixture of the ben-
zhydrazide and corresponding aldehyde/ketone in the presence
of 1–2 drops of glacial acetic acid in methanol for 4 h. Scheme 1
describes the syntheses of acylhydrazones.
nicotinoylhydrazone
(H2hman)
cal behavior of the complexes and free hydrazones was studied in
DMF solutions by cyclic voltammetry on a CHI 608D electrochemi-
cal analyzer with platinum disc as working electrode. The counter
electrode used was platinum wire and Ag/Ag+ the reference elec-
trode. The supporting electrolyte used was tetrabutylammonium
phosphate (TBAP).
3. Results and discussion
Elemental Anal. Found (Calcd.) (%):
H2hmbb: C, 66.08 (66.66); H, 5.47 (5.22); N, 10.26 (10.36).
H2hmab: C, 67.25 (67.59); H, 5.61 (5.67); N, 9.68 (9.85).
All the four complexes were synthesized in an identical way and
in all of them hydrazones act as tridentate dianionic ONO donor
ligands towards MoO2 centre. The experimental and calculated
2+
analytical data are in very close agreement. Magnetic susceptibil-
ity studies indicate diamagnetic nature of these complexes and
from this it is evident that Mo is in +6 oxidation state. The cyclic
voltammograms of the complexes did not show any redox prop-
erties. Reversible one electron reduction of [MoO2]2+ complexes to
analogous [MoO2]+ complexes is uncommon and occurs only when
aprotic ligands and solvents are used and when the ligand has suf-
ficient steric bulk to prevent dimerization [18]. So the absence of
redox behavior is an evidence for the dimeric nature of the com-
pounds.
2.2.2. Syntheses of complexes
The complexes [MoO2hmbn]2·H2O (1), [MoO2hman]2·H2O (2),
[MoO2hmbb]2·H2O (3) and [MoO2hmab]2 (4) were prepared in
a similar way by refluxing methanolic solutions of MoO2(acac)2
(1 mmol, 0.326 g) with H2hmbn·H2O (1 mmol, 0.289 g), H2hman
(1 mmol, 0.285 g), H2hmbb (1 mmol, 0.270 g) and H2hmab (1 mmol,
0.284 g) respectively. Initially on mixing, the solutions turn to an
orange color and after refluxing for 4 h the complexes separated.
All the four complexes are orange colored. Our attempts to get
single crystals suitable for X-ray diffraction studies were not suc-
cessful. Scheme 2 illustrates the syntheses of complexes and the
water molecules are omitted in the scheme.
3.1. Conductance measurements
The molar conductivity values for all the four complexes in
10−3 M DMF solutions are in the range of 2–5 ohm−1 cm2 mol−1
which is much less than the value of 65–90 ohm−1 cm2 mol−1
reported for a 1:1 electrolyte in the same solvent [19]. So the
conductance measurements in DMF suggest that they are non-
electrolytes.
Elemental Anal. Found (Calcd.) (%):
[MoO2hmbn]2·H2O (1): C, 41.50 (41.40); H, 2.29 (2.98); N, 10.57
(10.34).
[MoO2hman]2·H2O (2): C, 42.85 (42.87); H, 3.20 (3.36); N, 10.17
(10.00).
[MoO2hmbb]2·H2O (3): C, 44.87 (44.46); H, 3.64 (3.23); N, 6.57
(6.91).
3.2. Thermal studies
[MoO2hmab]2 (4): C, 46.14 (46.84); H, 3.32 (3.44); N, 6.86 (6.83).
The thermal analyses give information concerning the ther-
mal stability and nature of water molecules in complexes. Reports
show that the weight losses for lattice water are in the range of
50–130 ◦C [20,21]. The complexes 1, 2 and 3 showed loss of weight
which corresponds to one water molecule in the temperature range
80–110 ◦C indicate the presence of lattice water in these com-
plexes. In complex 4, there was no weight loss in the region below
250 ◦C which shows the absence of water molecule. All the com-
plexes showed weight losses in the temperature range 250–750 ◦C
due to the decomposition of ligands. Above 750 ◦C a plateau is
observed which corresponds to the formation of MoO3. TG–DTG
curves of complexes 1 and 4 are shown in Figs. 1 and 2.
2.3. Physical measurements
Elemental analyses of the acylhydrazones and their molybde-
num complexes were carried out using a Vario EL III CHNS analyzer
at SAIF, Kochi, India. Infrared spectra were recorded on a JASCO
FT/IR-4100 type A spectrometer in the range 4000–400 cm−1 using
KBr pellets. Electronic spectra were recorded on a Cary 5000 ver-
sion 1.09 UV–VIS–NIR spectrophotometer using acetonitrile as the
solvent. NMR spectra of the complexes and ligands were recorded
in DMSO solutions in a Bruker AMX 400 FT-NMR Spectrometer
using TMS as the internal standard at the Sophisticated Instruments
Facility, Indian Institute of Science, Bangalore, India. Magnetic sus-
ceptibility studies were done with Gouy balance. TG–DTG analyses
of the prepared complexes were carried out on a Perkin Elmer,
Diamond thermogravimetric analyzer. The heat flow used was
10 ◦C/min under nitrogen atmosphere over a temperature range
of 50–1000 ◦C. The molar conductivities were determined for all
the four complexes in 10−3 M DMF solutions. The electrochemi-
3.3. Infrared and electronic spectral studies
The most interesting vibrational frequencies of acylhydrazones
and their molybdenum complexes, which help us to understand
the coordination environment of the metal in these complexes, are
given in Table 1. In the IR spectra of the free hydrazones, bands due

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N. Mathew, M.R.P. Kurup / Spectrochimica Acta Part A 78 (2011) 1424–1428
Table 1
Selected IR bands (cm−1) with tentative assignments of Mo(VI) complexes.
Compound
␯(C O)
␯(C N)
␯(N C)
␯(N–N)
␯(C–O)
␯(Mo O)
␯(Mo O–Mo)
H2hmbn·H2O
1643
–
1638
–
1630
–
1650
–
1604
1589
1602
1593
1600
1596
1601
1597
–
1540
–
1532
–
1535
–
1549
1110
1129
1135
1140
1130
1138
1127
1156
1284
1225
1262
1249
1286
1230
1265
1234
–
[MoO2hmbn]2·H2O (1)
903, 937
–
913, 942
–
910, 930
–
833
854
850
854
H2hman
[MoO2hman]2·H2O (2)
H2hmbb
[MoO2hmbb]2·H2O (3)
H2hmab
[MoO2hmab]2 (4)
913, 942
Fig. 1. TG–DTG plot of [MoO2hmbn]2·H2O (1).
Fig. 2. TG–DTG plot of [MoO2hmab]2 (4).
to carbonyl group are observed in the region 1630–1650 cm−1 and
these are absent in the spectra of the complexes which suggests
enolization of ligands. This is also confirmed from the fact that the
O–H and N–H stretching frequencies observed around 3220 and
3035 cm−1 in free hydrazones were found to be absent in com-
plexes. Another result obtained from this is that the ligands are
coordinated in a dianionic form in these complexes.
The azomethine bands of the acylhydrazones found in the range
1600–1605 cm−1 are shifted to lower frequencies by 4–15 cm−1
in the spectra of the complexes, indicating the coordination of
azomethine nitrogen to the metal [22]. The increase in ␯(N–N)
bands in complexes, due to the increase in double bond character
confirm the coordination of the ligands through the azomethine
nitrogen [23]. The ␯(C–O) bands present in ligands are shifted to
lower frequencies in complexes suggesting the coordination of the
phenolic oxygen. In all complexes two bands are observed in the
IR spectra of all compounds except 4 showed broad bands
around 3430 cm−1 which indicate the presence of lattice water.
X
H3C
O
H3C
O
reflux 4 h
solvent-methanol
+
[MoO2(acac)2]
O
N
N
OH
O
R
Mo
O
O
N
R
O
O
O
Mo
O
NH
N
N
R
O
O
X
CH3
X
A
R = H & X=N; A = Complex 1
R = CH3 & X= N; A = Complex 2
R = H & X=C; A = Complex 3
R = CH3 & X=C; A= Complex 4
Scheme 2. Syntheses of complexes.

N. Mathew, M.R.P. Kurup / Spectrochimica Acta Part A 78 (2011) 1424–1428
1427
Fig. 5. 1H NMR spectrum of H2hmbn·H2O.
Fig. 3. Electronic spectra of H2hmbn·H2O and [MoO2hmbn]2·H2O.
region 900–950 cm−1 which can be assigned to symmetric and anti-
2+
symmetric vibrations of the cis-MoO2 core [24]. As per previous
reports structurally characterized dimeric dioxomolybdenum(VI)
complexes of tridentate ligands have sharp bands in the region
800–850 cm−1 [25,26]. Here the presence of a band in the above
said region indicates the formation of dimeric dioxomolybdenum
complexes.
The electronic spectra of all the four acylhydrazones showed
bands in the region 30,500–42,000 cm−1 due to ␲–␲* transitions.
The n–␲* transitions that are forbidden by the selection rules are
not observed in these compounds. In the electronic spectra of com-
plexes these transitions are slightly shifted. Fig. 3 illustrates the
shift in transitions of one of the ligands, H2hmbn·H2O on com-
plexation. The complexes show strong bands ∼24,400 cm−1 and
are assigned as ligand to metal charge transfer transitions [25–27].
Electronic spectra of the complexes are presented in Fig. 4 and the
assignments are listed in Table 2.
Fig. 6. 1H NMR spectrum of H2hman.
pounds. Because of their diamagnetic nature, the complexes were
studied by NMR spectroscopy. The 1H NMR spectra of the ligands
and complexes have been recorded with DMSO as solvent. In the
spectra of the free hydrazones there are sharp singlets in the region
12–14 ppm with an area integral of one which is due to phenolic
OH protons. They also gave singlets in the range 11–12 ppm due
to another proton showing the existence of iminol form in solu-
tion. Large ı values of these protons may be due to intramolecular
hydrogen bonding. Upon D2O exchange, the intensity of these sig-
nals significantly decreases, which suggests that these protons are
easily exchangeable and confirm the assignment. The singlets with
an area integral of three in the range of 3–3.8 ppm indicate the
presence of three methoxy hydrogens. Peaks for aromatic protons
were found in the region 6–8 ppm. Peaks corresponding to OH and
NH protons were absent in the spectra of complexes and this is a
clear cut evidence for the coordination of ligands in the enolic form,
i.e. from NMR spectra, the coordination of ligands in the dianionic
form is confirmed. The peaks corresponding to the aromatic pro-
3.4. NMR spectral studies
Proton Nuclear Magnetic Resonance (1H NMR) Spectroscopy is
a powerful tool used for the determination of the structure of com-
Table 2
Electronic spectral assignments (cm−1) of Mo(VI) complexes.
Compound
Intraligand transitions
Charge transfer
transition
[MoO2hmbn]2·H2O (1)
[MoO2hman]2·H2O (2)
[MoO2hmbb]2·H2O (3)
[MoO2hmab]2 (4)
31,360, 40,910
24,230
24,150
24,230
24,970
31,580, 39,600, 42,840
31,640, 39,740, 42,830
31,640, 40,360, 42,550
Fig. 4. Electronic spectra of the complexes.

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N. Mathew, M.R.P. Kurup / Spectrochimica Acta Part A 78 (2011) 1424–1428
physicochemical techniques. Conductivity values showed the non-
electrolytic nature of complexes, and magnetic studies revealed
diamagnetic nature showing the existence of +6 oxidation state of
Mo in these complexes. Spectral studies help us to conclude that all
complexes are dimeric in nature and in all cases ligands are in tri-
coordinate dideprotonated forms. Thermal stability investigations
of complexes enable us to evaluate the assumed positions of the
water molecules in the inner or outer sphere of coordination.
Acknowledgements
Nancy Mathew is thankful to the University Grants Commission,
New Delhi, India for the award of Senior Research Fellowship. The
authors are thankful to the Sophisticated Analytical Instrumenta-
tion Facility, Cochin University of Science and Technology, Kochi
22, India for elemental analyses and IISc Bangalore for providing
NMR spectra.
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Four dioxomolybdenum complexes of four different ONO donor
hydrazones were synthesized and characterized using various