Synthesis, structural studies and catalytic activity of dioxidomolybdenum(VI) complexes with aroylhydrazones of naphthol-derivative

Article abstract of DOI:10.1016/j.poly.2013.08.055

Reaction of the salicylhydrazone of 2-hydroxy-1-naphthaldehyde (H ₂L¹), anthranylhydrazone of 2-hydroxy-1-naphthaldehyde (H₂L²), benzoylhydrazone of 2-hydroxy-1-acetonaphthone (H₂L³) and anthranylhydrazone of 2-hydroxy-1- acetonaphthone (H₂L⁴; general abbreviation H₂L) with [MoO₂(acac)₂] afforded a series of 5- and 6- coordinate Mo(VI) complexes of the type [MoO₂L^1-2(ROH)] [where R = C₂H₅ (1) and CH₃ (2)], and [MoO ₂L^3-4] (3 and 4). The substrate binding capacity of 1 has been demonstrated by the formation of one mononuclear mixed-ligand dioxidomolybdenum complex [MoO₂L¹(Q)] {where Q = γ-picoline (1a)}. Molecular structure of all the complexes (1, 1a, 2, 3 and 4) is determined by X-ray crystallography, demonstrating the dibasic tridentate behavior of ligands. All the complexes show two irreversible reductive responses within the potential window -0.73 to -1.08 V, due to Mo ^VI/Mo^V and Mo^V/Mo^IV processes. Catalytic potential of these complexes was tested for the oxidation of benzoin using 30% aqueous H₂O₂ as an oxidant in methanol. At least four reaction products, benzoic acid, benzaldehyde-dimethylacetal, methyl benzoate and benzil were obtained with the 95-99% conversion under optimized reaction conditions. Oxidative bromination of salicylaldehyde, a functional mimic of haloperoxidases, in aqueous H₂O₂/KBr in the presence of HClO₄ at room temperature has also been carried out successfully.

Full text of DOI:10.1016/j.poly.2013.08.055

Polyhedron 67 (2014) 1–10
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, structural studies and catalytic activity
of dioxidomolybdenum(VI) complexes with aroylhydrazones
of naphthol-derivative
b
Sagarika Pasayat a, Subhashree P. Dash a, Satabdi Roy a, Rupam Dinda a, , Sarita Dhaka ,
⇑
Mannar R. Maurya b, Werner Kaminsky c, Yogesh P. Patil d, M. Nethaji d
a Department of Chemistry, National Institute of Technology, Rourkela 769008, Odisha, India
b Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India
c Department of Chemistry, University of Washington, Box 351700, Seattle, WA 98195, USA
d Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
a r t i c l e i n f o
a b s t r a c t
Reaction of the salicylhydrazone of 2-hydroxy-1-naphthaldehyde (H2L1), anthranylhydrazone of 2-
hydroxy-1-naphthaldehyde (H2L2), benzoylhydrazone of 2-hydroxy-1-acetonaphthone (H2L3) and
anthranylhydrazone of 2-hydroxy-1-acetonaphthone (H2L4; general abbreviation H2L) with [MoO2(-
acac)2] afforded a series of 5- and 6- coordinate Mo(VI) complexes of the type [MoO2L1–2(ROH)] [where
R = C2H5 (1) and CH3 (2)], and [MoO2L3–4] (3 and 4). The substrate binding capacity of 1 has been dem-
onstrated by the formation of one mononuclear mixed-ligand dioxidomolybdenum complex [MoO2L1(Q)]
Article history:
Received 20 May 2013
Accepted 22 August 2013
Available online 3 September 2013
Keywords:
Aroylhydrazones
Naphthol-derivative
{where Q = c-picoline (1a)}. Molecular structure of all the complexes (1, 1a, 2, 3 and 4) is determined by
X-ray crystallography, demonstrating the dibasic tridentate behavior of ligands. All the complexes show
two irreversible reductive responses within the potential window ꢀ0.73 to ꢀ1.08 V, due to MoVI/MoV and
MoV/MoIV processes. Catalytic potential of these complexes was tested for the oxidation of benzoin using
30% aqueous H2O2 as an oxidant in methanol. At least four reaction products, benzoic acid, benzaldehyde-
dimethylacetal, methyl benzoate and benzil were obtained with the 95–99% conversion under optimized
reaction conditions. Oxidative bromination of salicylaldehyde, a functional mimic of haloperoxidases, in
aqueous H2O2/KBr in the presence of HClO4 at room temperature has also been carried out successfully.
Ó 2013 Elsevier Ltd. All rights reserved.
Dioxidomolybdenum(VI) complexes
X-ray crystal structure
Catalytic oxidation of benzoin
Oxidative bromination
1. Introduction
Accordingly, we have synthesized and examined a number of tran-
sition metal-Schiff base compounds using aroylhydrazone as
Schiff bases, characterized by the azomethine group (–RC@N–),
form a significant class of compounds in medicinal and pharma-
ceutical chemistry and are known to have biological applications
due to their antibacterial [1–6], antifungal [3–6] and antitumor
[7,8] activity. The incorporation of transition metals into these
compounds leads to the enhancement of their biological activities
and decrease in the cytotoxicity of both the metal ion and Schiff
base ligand [9–11]. On the other hand, aroylhydrazones are excel-
lent multidentate ligands for transition metals. They have been
shown to exhibit a range of biological e.g. antiamoebic activity
[12] and DNA synthesis inhibition or antiproliferative behavior
[13–15]. For quite some time, we have been engaged in a program
aimed at credible synthesis of oxygen and nitrogen co-ordinated
transition metal compounds in order to approach the biological
co-ordination units as potential lead molecules for drug designing.
tridentate (ONO) – donor ligand [16–18]. Their properties can be
tuned by modification of either the aromatic aldehyde or the
hydrazide component. Our aim was to increase lipophilicity using
bulky aromatic groups and to investigate the influence on the coor-
dination sphere as well as reactivity by the use of aroylhydrazone
of naphthol-derivatives.
The coordination chemistry of molybdenum has become a
fascinating area of research in recent years because of the presence
of molybdenum in metalloenzymes [19–25]. Catalytic applications
of molybdenum complexes in organic transformations, particularly
in the epoxidation of alkenes have been explored much as evi-
denced by number of publications [26–39]. The catalytic activity
of molybdenum complexes are sensitive to the donor/acceptor
ability of the ligand, and to steric and strain factors. Therefore,
we have designed sterically hindered/bulky ONO donor aroylhyd-
razones of naphthol-derivative (Scheme 1) in the light of above
factors to prepare dioxidomolybdenum(VI) complexes. Varying
the steric bulk of the aroylhydrazone ligands also controls the
⇑
Corresponding author. Tel.: +91 661 246 2657; fax: +91 661 246 2022.
E-mail address: rupamdinda@nitrkl.ac.in (R. Dinda).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.08.055

2
S. Pasayat et al. / Polyhedron 67 (2014) 1–10
%Conversion of substrate ¼ 100
Peak area of a substrate
R1
ꢀ
Total area of substrate þ products
R
ꢁ 100
NH
N
HO
Peak area of a product
Total area of products
%Selectivity of a product ¼
ꢁ 100
O
R1 = H , R = OH, H2L1
R1 = H , R = NH2, H2L2
2.3. Synthesis of Ligands (H2L1–4
)
Schiff base ligands, H2L1–4 were prepared by the condensation
of carbonyl compounds and the respective acidhydrazide in equi-
molar ratio in ethanol medium by a standard procedure [43]. The
resulting yellowish- white compounds were filtered, washed with
ethanol and dried over fused CaCl2.
R1 = CH3, R = H, H2L3
R1 = CH3, R = NH2, H2L4
H2L1: Yield: 0.22 g (72%). Anal. Calc. for C18H14N2O3: C, 70.58; H,
4.57; N, 9.15. Found: C, 70.59; H, 4.54; N, 9.12%. 1H NMR (DMSO-
d6,400 MHz) d;12.74 (s, 1H, naphthyl-OH), 12.07 (s, 1H, NH),
11.87 (s, 1H, aryl-OH), 9.54 (s, 1H, HC@N), 8.33–6.62 (m, 10H, Aro-
matic) ppm. 13C NMR (DMSO-d6, 100 MHz) d; 163.72, 158.51,
157.89, 147.44, 133.78, 132.68, 131.44, 128.69, 128.55, 127.57,
127.50, 123.33, 120.73, 118.92, 118.65, 117.05, 115.48, 108.36.
H2L2: Yield: 0.19 g (62%). Anal. Calc. for C18H15N3O2: C, 70.81; H,
4.90; N, 13.77. Found: C, 70.78; H, 4.92; N, 13.75%. 1H NMR (DMSO-
d6, 400 MHz) d; 12.97 (s, 1H, naphthyl-OH), 12.04 (s, 1H, NH), 9.47
(s, 1H, HC@N), 8.18–6.64 (m, 10H, Aromatic), 6.61 (s, 2H, NH2)
ppm. 13C NMR (DMSO-d6, 100 MHz) d; 165.12, 158.33, 150.93,
146.33, 133.18, 132.91, 132.07, 129.45, 128.57, 128.25, 128.16,
123.96, 120.88, 119.42, 117.12, 115.14, 112.62, 109.06.
Scheme 1. Schematic representation of Ligand.
coordination number of the molybdenum in these complexes.
Novel structural features, reactivity patterns of these complexes
and their catalytic activity for the oxidation of benzoin are reported
here. Selective transformation of
a-hydroxyketone to the corre-
sponding -diketone is one of the most important fundamental
a
reactions in organic chemistry [40,41]. The catalytic potential of
dioxidomolybdenum(VI) complexes for the oxidative bromination
of salicylaldehyde, as functional mimic of haloperoxidase has also
been explored.
H2L3: Yield: 0.18 g (60%). Anal. Calc. for C19H16N2O2: C, 74.89; H,
5.09; N, 9.10. Found: C, 74.90; H, 5.07; N, 9.11%. 1H NMR (DMSO-
d6,400 MHz) d; 10.31 (s, 1H, naphthyl-OH), 9.55 (s, 1H, NH),
7.91–7.27 (m, 11H, Aromatic), 2.34 (s, 3H, CH3) ppm. 13C NMR
(DMSO-d6,100 MHz) d; 153.71, 152.63, 134.24, 131.82, 131.51,
130.21, 129.06, 128.76, 128.36, 128.23, 127.86, 127.43, 126.99,
123.67, 123.19, 118.99, 118.71, 113.74, 24.30.
2. Experimental
2.1. Materials
[MoO2(acac)2] was prepared as described in the literature
[42]. Reagent grade solvents were dried and distilled prior to
use. All other chemicals were reagent grade, available commer-
cially and used as received. Commercially available TBAP (tetra
butyl ammonium perchlorate) was dried and used as a support-
ing electrolyte for recording cyclic voltammograms of the
complexes.
H2L4: Yield: 0.21 g (65%). Anal. Calc. for C19H17N3O2: C, 71.47; H,
5.32; N, 13.16. Found: C, 71.45; H, 5.33; N, 13.15%. 1H NMR (DMSO-
d6, 400 MHz) d; 10.35 (s, 1H, naphthyl-OH), 9.17 (s, 1H, NH), 7.91–
6.23 (m, 10H, Aromatic), 6.05 (s, 2H, NH2), 2.35 (s, 3H, CH3) ppm.
13C NMR (DMSO-d6, 100 MHz) d; 153.98, 152.47, 152.36, 149.84,
132.47, 131.56, 130.22, 129.09, 128.34, 127.98, 127.67, 123.78,
123.14, 118.94, 117.04, 115.46, 114.69, 113.59, 24.15.
2.2. Physical measurements
Elemental analyses were performed on a Vario ELcube CHNS
Elemental analyzer. IR spectra were recorded on a Perkin–Elmer
Spectrum RXI spectrometer. 1H NMR spectra were recorded with
a Bruker Ultra shield 400 MHz spectrometer using SiMe4 as an
internal standard. Electronic spectra were recorded on a Lamda25,
PerkinElmer spectrophotometer. Electrochemical data were col-
lected using a PAR electrochemical analyzer and a PC-controlled
Potentiostat/Galvanostat (PAR 273A) at 298 K in a dry nitrogen
atmosphere. Cyclic voltammetry experiments were carried out
with a platinum working electrode, platinum auxiliary electrode,
Ag/AgCl as reference electrode and TBAP as supporting electrolyte.
A Shimadzu 2010 plus gas-chromatograph fitted with an Rtx-1
2.4. Synthesis of complexes (1–4)
These complexes were prepared modifying
a previously
published procedure [44–46]. To a refluxing solution of ligand,
H2L1–4 (1.0 mmol) in 30 mL of alcohol {ethanol (1 and 2) and meth-
anol (3 and 4)}, 1.0 mmol of MoO2(acac)2 was added. The color of
the solution changed to dark red. The mixture was then refluxed
for 3 h. After leaving the solution for 2 days at room temperature,
fine red colored crystals were isolated. Crystals of most complexes
were suitable for single crystal X-ray analysis.
[MoO2L1 (C2H5OH)](1):Yield: 0.29 g (60%). Anal. Calc. for C20H18-
N2O6Mo: C, 50.20; H, 3.76; N, 5.85. Found: C, 50.18; H, 3.77; N,
5.84%. 1H NMR (DMSO-d6, 400 MHz) d; 11.41 (s, 1H, aryl-OH),
10.07 (s, 1H, HC@N), 8.61–6.99 (m, 10H, Aromatic), 4.36 (s, 1H,
OH-ethanol), 3.42 (q, 2H, CH2-ethanol), 1.05 (s, 3H, CH3-ethanol)
ppm. 13C NMR (DMSO-d6, 100 MHz) d; 168.19, 160.20, 158.59,
152.76, 136.14, 134.01, 132.51, 129.03, 128.99, 128.89, 128.49,
124.91, 121.87, 120.25, 119.38, 117.10, 113.21, 111.65, 56.00,
18.54.
capillary column (30 m ꢁ 0.25 mm ꢁ 0.25
lm) and a FID detector
was used to analyze the reaction products and their quantifications
were made on the basis of the relative peak area of the respective
product. The identity of the products was confirmed using a GC–
MS model Perkin–Elmer, Clarus 500 and comparing the fragments
of each product with the library available. The percent conversion
of substrate and selectivity of products was calculated from GC
data using the formulae:

S. Pasayat et al. / Polyhedron 67 (2014) 1–10
3
[MoO2L2(CH3OH)] (2): Yield: 0.23 g (53%). Anal. Calc. for C19H17-
N3O5Mo: C, 49.24; H, 3.67; N, 9.07. Found: C, 49.25; H, 3.64; N,
9.08%. 1H NMR (DMSO-d6, 400 MHz) d; 9.92 (s, 1H, HC@N), 8.57–
6.62 (m, 10H, Aromatic), 7.12 (s, 2H, NH2), 4.10 (s, 1H, OH-metha-
nol), 3.15 (s, 3H, CH3- methanol) ppm. 13C NMR (DMSO-d6,
100 MHz) d; 169.06, 160.58, 151.88, 150.05, 135.93, 133.56,
132.91, 130.33, 129.44, 129.25, 128.77, 125.15, 122.16, 120.86,
116.51, 115.18, 112.26, 109.63, 49.07.
125.11, 122.35, 120.73, 119.85, 117.58, 113.71, 112.15, 56.50,
20.88, 19.02.
2.6. Crystallography
Suitable single crystal of 1, 1a and 2–4 was chosen for X-ray
diffraction studies. Crystallographic data and details of refinement
are given in Table 1. The unit cell parameters and the intensity data
for the complexes (1, 2, 3 and 4) were collected at ꢂ293 K, on a
Bruker Smart Apex CCD diffractometer and complex (1a) was col-
lected at 100 K, on a Bruker Smart Apex II CCD diffractometer using
[MoO2L3](3): Yield: 0.26 g (54%). Anal. Calc. for C19H14N2O4Mo:
C, 66.97; H, 3.25; N, 6.51. Found: C, 66.95; H, 3.26; N, 6.50%. 1H
NMR (DMSO-d6, 400 MHz) d; 8.11–7.18 (m, 11H, Aromatic), 2.93
(s, 3H, CH3), ppm. 13C NMR (DMSO-d6,100 MHz) d; 169.36,
165.06, 163.01, 152.34, 134.25, 132.43, 131.88, 130.98, 130.13,
129.24, 128.52, 127.84, 126.07, 125.02, 120.38, 119.84, 116.46,
113.69, 24.23.
graphite monochromated Mo
Ka radiation (k = 0.71073 Å),
employing the -2h scan techniques. The intensity data were cor-
x
rected for Lorentz, polarization and absorption effects. The struc-
tures were solved using the SHELXS97 [47] and refined using the
SHELXL97 [48] computer programs. The non-hydrogen atoms were
refined anisotropically.
[MoO2L4](4): Yield: 0.21 g (52%). Anal. Calc. for C19H15N3O4Mo:
C, 51.23; H, 3.37; N, 9.43. Found: C, 51.24; H, 3.35; N, 9.44%. 1H
NMR (DMSO-d6, 400 MHz) d; 8.01–6.87 (m, 10H, Aromatic), 7.04
(s, 2H, NH2), 2.86 (s, 3H, CH3) ppm. 13C NMR (DMSO-d6,100 MHz)
d; 170.36, 162.91, 162.82, 150.17, 134.03, 133.14, 131.87, 130.36,
130.07, 129.37, 127.77, 126.08, 124.96, 120.38, 119.82, 116.53,
115.47,110.15, 24.40.
2.7. Catalytic reactions
2.7.1. Oxidation of benzoin
In a typical oxidation reaction, benzoin (1.06 g, 5 mmol), aque-
ous 30% H2O2 (1.71 g, 15 mmol) and catalyst (0.0005 g) were
mixed in methanol (10 mL). The reaction mixture was heated un-
der reflux with stirring for 4 h. The progress of the reaction was
monitored by withdrawing samples at different time intervals
and samples were extracted with n-hexane and then analyzed
quantitatively by gas chromatography. The effect of various
parameters such as amount of catalyst, amount of oxidant, and sol-
vent were checked to optimize the conditions for the best perfor-
mance of the catalyst. The identity of the products was
confirmed by GC-mass.
2.5. Synthesis of mixed-ligand complex [{MoO2L1(Q)}, where Q =
c-
picoline (1a)]
To a clear red solution of 1 (0.50 mmol) in CH3CN (30 mL), c-
picoline (0.73 mmol) was added and the reaction mixture was
refluxed for 3 h. Slow evaporation of the red filtrate over 2 days
produced dark red crystals. These were separated by filtration.
Yield 0.25 g (57%). Anal. Calc. for C24H19N3O5Mo: C, 54.85; H,
3.61; N, 8.00. Found: C, 54.86; H, 3.59; N, 8.01%. 1H NMR (DMSO-
d6, 400 MHz) d; 11.42 (s, 1H, aryl-OH), 10.08 (s, 1H, HC = N),
8.62–6.99 (m, 10H, Aromatic), 7.24–6.97 (m, 4H, Aromatic-
c
-pico-
2.7.2. Oxidative bromination of salicylaldehyde
line), 2.31 (s, 3H, CH3-
c
-picoline) ppm. 13C NMR (DMSO-
Salicylaldehyde (0.610 g, 5 mmol) was added to an aqueous
solution (20 mL) of KBr (1.785 g, 15 mmol), followed by addition
of aqueous 30% H2O2 (1.71 g, 15 mmol) in a 100 mL reaction flask.
d6,100 MHz) d; 168.69, 160.70, 159.09, 149.76, 147.22, 136.62,
134.49, 133.00, 129.52, 129.52, 129.47, 129.38, 128.97, 125.38,
Table 1
Crystal and refinement data of complexes 1, 1a, 2, 3 and 4.
Compound
1
1a
2
3
4
Formula
M
C
20H18N2O6Mo
C24H19N3O5Mo
525.36
C19H17N3O5Mo
463.30
C19H14N2O4Mo
430.26
C19H15N3O4Mo
445.28
478.30
Crystal symmetry
Space group
a (Å)
b (Å)
c (Å)
triclinic
P1
monoclinic
P21/c
13.313(2)
8.0476(14)
21.357(3)
90
111.844(11)
90
2123.9(6)
4
triclinic
P1
monoclinic
P21/c
4.0623(12)
25.842(8)
15.761(5)
90
98.332(16)
90
1637.1(9)
4
monoclinic
P21/n
7.8923(3)
13.8894(5)
15.3314(6)
90
100.355(2)
90
1653.24(11)
4
ꢀ
ꢀ
7.8196(2)
11.1222(3)
11.4265(3)
80.0650(10)
85.1030(10)
82.1190(10)
967.69(4)
2
8.1198(5)
9.9276(6)
12.3349(8)
101.8430(10)
103.7180(10)
91.6650(10)
942.23(10)
2
a
(°)
b (°)
c
(°)
V (Å3)
Z
Dcalc (g cmꢀ3
F(000)
)
1.642
484
0.718
1.643
1064
0.661
1.633
468
0.732
1.746
864
0.830
1.789
896
0.827
l
(Mo K
a
) (mmꢀ1
)
Max./min. trans.
2h (max) (°)
0.9514/0.8999
26.00
0.9934/0.9804
25.42
0.9370/0.8981
26.00
0.9755/0.9366
25.00
0.9918/0.9598
25.99
Reflections collected/
15934/3799
61361/3914
9689/3671
25568/2871
26992/3249
unique
R1a[I > 2
r
(I)]
R1 = 0.0274,
wR2 = 0.0685
R1 = 0.0313, wR2 = 0.0715 R1 = 0.1345,
wR2 = 0.1390
1.038
R1 = 0.0746,
wR2 = 0.1208
R1 = 0.0249,
wR2 = 0.0668
R1 = 0.0267,
wR2 = 0.0680
1.095
R1 = 0.0654,wR2 = 0.1639
R1 = 0.0396,
wR2 = 0.0943
b
wR2 [all data]
R1 = 0.0740, wR2 = 0.1688 R1 = 0.0641,
wR2 = 0.1059
1.174
1.855/ꢀ1.795
S [Goodness of fit]
1.086
1.225/ꢀ0.732
1.089
0.530/ꢀ0.567
Min./max. res.(e.Åꢀ3
)
0.680/ꢀ0.427
0.341/ꢀ0.229
P
P
a
R1
=
|FO| ꢀ |FC/ |FO|.
P
P
wR2 = { [w(FO ꢀ FC2)2]/ [w(FO2)2]}
.
b
2
½

4
S. Pasayat et al. / Polyhedron 67 (2014) 1–10
The catalyst (0.0005 g) and 70% HClO4 (0.715 g, 5 mmol) were
added, and the reaction mixture was stirred at room temperature
(20 °C). Three additional 5 mmol portions of 70% HClO4 were
further added to the reaction mixture in three equal portions at
45 min intervals under continuous stirring. After 3 h, the separated
white products were extracted with CH2Cl2 and dried. The crude
mass was dissolved in methanol and was subjected to gas chroma-
tography. The identity of the products was confirmed as mentioned
above.
All these complexes are highly soluble in aprotic solvents, viz.
DMF or DMSO and are sparingly soluble in alcohol, CH3CN and
CHCl3. All these complexes are diamagnetic, indicating the
presence of molybdenum in the +6 oxidation state, and are non-
conducting in solution.
Spectral characteristics of compounds are listed in Table 2. The
DMSO solutions of all the complexes display a medium intensity
band in the 468–423 nm region (Fig. S1) and two strong absorp-
tions in the 340–256 nm range, which are assignable to ligand to
molybdenum (pp–dp) charge transfer (LMCT) and intraligand
transitions, respectively [16,49,50].
3. Results and discussion
IR spectra of the complexes contain all the pertinent bands of
the coordinated tridentate ligands [16,49,50]. In addition, 1, 1a, 2
and 4 display two strong peaks in the range 937–902 cmꢀ1 due
3.1. Synthesis and spectral properties
to terminal
m(M@Ot) stretching [16,49–51]. There are two
Reactions of the selected aroylhydrazones (c.f. Scheme 1) with
[MoO2(acac)2] in refluxing alcohol afforded two different types of
MoVI complexes, [MoO2L(ROH)] and [MoO2L]; Equations 1 and 2.
The formation of mixed-ligand mononuclear complex [MoO2L(Q)]
relatively strong and broad peaks around 821–863 cmꢀ1 due to
weakened (Mo – O ? Mo) for oligomeric complex 3, observed.
The details of IR spectra of the free ligands and their corresponding
dioxidomolybdenum (VI) complexes are given in Table 2.
The 1H and 13C NMR (DMSO-d6) data of all the free ligands and
their corresponding dioxidomolybdenum(VI) complexes are given
in the experimental section. The spectra of the free ligands exhibit
a resonance at d = 12.97–10.31 due to naphthyl –OH (H2L1–4), at
d = 9.54–9.47 due to –HC@N protons (H2L1–2) and at d = 2.35–
2.34 due to –CH3 protons (H2L3–4), respectively. All the aromatic
protons of ligands are clearly observed in the expected region
d = 8.33–6.23. In the NMR spectra of complexes, the absence of sig-
nal due to aromatic (naphthyl) –OH indicates that the phenolic
group is coordinated to the metal centre after proton replacement
[16,49,50]. Similarly, the absence of signal due to –NH proton in
the complexes suggests that the ligands are coordinated to the me-
tal center via enolic form.
{where Q =
c-picoline (1a)} has been achieved by the reaction of
1
with -picoline; Equation 3. Proposed structures of these
c
complexes (Scheme 2) are based on their spectroscopic character-
ization (IR, electronic and 1H NMR spectroscopy), elemental
analyses and single crystal X-ray diffraction studies. The ligands
coordinate through their dianionic (ONO)2ꢀ enolate tautomeric
forms.
½MoO2ðacacÞ2ꢃ þ H2L þ ROH ! ½MoO2LðROHÞꢃ þ 2Hacac
½H2L ¼ H2L1; R ¼ C2H5 : ð1Þ and H2L ¼ H2L2; R ¼ CH3 : ð2Þꢃ
ð1Þ
ð2Þ
ð3Þ
½MoO2ðacacÞ2ꢃ þ H2L ! ½MoO2Lꢃn þ 2Hacac
½H2L ¼ H2L3; n ¼ 1 : ð3Þ and H2L ¼ H2L4; n ¼ 1 : ð4Þꢃ
½MoO2L1ðC2H5OHÞꢃ þ Q ! ½MoO2L1Qꢃ þ C2H5OH
3.2. Electrochemical properties
½Q ¼
c
ꢀ picoline : ð1aÞꢃ
Electrochemical properties of the complexes have been studied
by cyclic voltammetry in DMF solution (0.1 M TBAP). Voltammetric
data are given in Table 3. The CV traces of the complexes exhibit
two irreversible reductive responses within the potential window
ꢀ0.73 to ꢀ1.08 V, which are assigned to MoVI/MoV and MoV/MoIV
processes respectively. The lack of anodic response, even at a high-
er scan rate, is clearly due to rapid decomposition of the reduced
species [16,49,50].
R1
N
R
N
3.3. X-ray structure of complexes
O
Complexes
1 and 2 are isostructural cis-dioxidomolybde-
O
num(VI) species; hence, details of 2 are not discussed here. Only
complex 1 as a representative case is discussed here. Selected bond
angles and bond lengths for both are given in Table 4 for compar-
ison purposes. The molecular structure and the atom numbering
scheme of complex 2 is shown in Fig. S2.
O
Mo
X
O
3.3.1. Description of the X-ray structure of complex 1 and 3
Ligand
X
Complex
The molecular structure and the atom numbering scheme for
the complex [MoO2L1(C2H5OH)] (1) and [MoO2L3] (3) are shown
in Fig. 1 and Fig. S3 respectively; the relevant bond distances and
angles are collected in Table 4. The coordination geometry around
the molybdenum (VI) atom in 1 and 3 reveals a distorted octahe-
dral environment with an NO5 coordination sphere (Scheme 2).
Each ligand molecule behaves as a dianionic tridentate one and
bonded to the metal centre through the phenolate oxygen O(1),
the enolate oxygen O(2) and the imine nitrogen N(1). In both the
complexes, one of the two oxo group O(4) is located trans to the
imine nitrogen in the same plane. For complex 1 the other oxo
group O(3) is located in the axial plane with the solvent molecule,
[MoO2L1 (C2H5OH)] (1)
[MoO2L1 (γ -pic)] (1a)
[MoO2L2(CH3OH)] (2)
[MoO2L3] (3)
H2L1
H2L1
H2L2
H2L3
H2L4
C2H5OH
γ-picoline
CH3OH
--
[MoO2L4] (4)
--
Scheme 2. Possible bonding modes of Mo(VI) complexes.

S. Pasayat et al. / Polyhedron 67 (2014) 1–10
5
Table 2
Characteristic IRa bands (
m
in cmꢀ1) and electronic spectral datab for the studied complexes (1, 1a and 2–4).
Complex
m
(C@O)
m
(C@N)
m(N–N)
m
(Mo@O)
kmax (nm) (e )
/dm3 molꢀ1 cmꢀ1
H2L1
H2L2
H2L3
H2L4
1621
1637
1652
1652
–
–
–
–
–
1610
1602
1622
1619
1602
1595
1597
1594
1590
–
–
–
–
1038
975
1021
1045
1049
–
–
–
–
–
–
–
–
[MoO2L1(C2H5OH)] (1)
937, 913
923, 902
931, 902
863, 821
935, 908
447 (5890), 340 (1591), 257 (1915)
446 (5505), 339 (1361), 257 (1770)
468 (51649), 329 (12711), 256 (13024)
423 (10701), 329 (23470), 258 (20373)
442 (10433), 329 (14886), 258 (27667)
[MoO2L1(
c
-pic)] (1a)
[MoO2L2(CH3OH)] (2)
[MoO2L3] (3)
[MoO2L4] (4)
a
In KBr pellet.
In DMSO.
b
Table 3
Cyclic voltammetric results for dioxidomolybdenum (VI) complexes (1, 1a and 2–4) at
298 K.
Complex
Epc [V]a
[MoO2L1(C2H5OH)] (1)
ꢀ0.81, ꢀ1.03
ꢀ0.79, ꢀ1.06
ꢀ0.73, ꢀ1.08
ꢀ0.75, ꢀ1.07
ꢀ0.77, ꢀ1.06
[MoO2L1(
c-pic)] (1a)
[MoO2L2(CH3OH)] (2)
[MoO2L3] (3)
[MoO2L4] (4)
a
Solvent: DMF; working electrode: platinum; auxiliary electrode: platinum;
reference electrode: Ag/AgCl; supporting electrolyte: 0.1 M TBAP; scan rate: 50 mV/
s. Epc is the cathodic peak potential.
C2H5OH. Whereas in complex 3 along with the two oxo-oxygen,
O(3) and O(4), the three ONO donor points of the ligand complete
the five coordinate environment around the Mo(VI) acceptor
center, the disposition of the donor points being roughly square
pyramidal. The sixth coordination site, trans to the oxo-oxygen
O(3), is occupied by an oxo-oxygen O(3!) of the next neighboring
complex molecule and this pattern is repeated leading to a chain
of MoO2L3 molecules (Fig. 2). This may be visualized as an effect
of stacking of the complex molecules along the z-axis [52] The
length of the Mo–O(3!) bond (2.374 (5) Å) is considerably longer
Fig. 1. ORTEP diagram of [MoO2L1(C2H5OH)], 1 with atom labeling scheme. Thermal
ellipsoids have been drawn at 50% probability.
than the other Mo–O bonds, the longest of which [Mo–O(2)] is
1.980 (5) Å which suggests that this should be explained as
oligomeric structure where one of the oxygen of one molybdenum
weakly interacts with other. The bond between Mo and the
azomethine nitrogen in the complexes are within the range of
Table 4
Selected bond distances [Å] and bond angles [°] for Complexes 1, 1a, 2, 3 and 4.
Complex 1
Complex 1a
Complex 2
Complex 3
Complex 4
Bond lengths
Mo(1)–O(1)
Mo(1)–O(2)
Mo(1)–O(3)
Mo(1)–O(4)
Mo(1)–O(5)
Mo(1)–N(1)
Mo(1)–N(3)
Mo(1)–O(3)#1
Bond angles
1.918(2)
2.000(2)
1.694(2)
1.698(2)
2.385(2)
2.225(2)
–
1.917(5)
2.017(5)
1.697(4)
1.713(4)
–
2.222(4)
2.429(5)
–
1.925(1)
1.992(1)
1.693(2)
1.706(2)
2.297(2)
2.234(2)
–
1.894(5)
1.980(5)
1.702(5)
1.687(5)
–
2.233(5)
–
2.374(5)
1.923(3)
1.991(3)
1.690(3)
1.701(3)
–
2.222(3)
–
–
–
–
O(1)–Mo(1)–O(2)
O(1)–Mo(1)–O(3)
O(1)–Mo(1)–O(4)
O(1)–Mo(1)–O(5)
O(1)–Mo(1)–N(1)
O(2)–Mo(1)–O(3)
O(2)–Mo(1)–O(4)
O(2)–Mo(1)–O(5)
O(2)–Mo(1)–N(1)
O(3)–Mo(1)–O(4)
O(3)–Mo(1)–O(5)
O(3)–Mo(1)–N(1)
O(4)–Mo(1)–O(5)
O(4)–Mo(1)–N(1)
O(5)–Mo(1)–N(1)
O(3)–Mo(1)–O(3)#1
149.48(8)
99.77(9)
102.67(9)
80.25(7)
80.55(7)
97.25(9)
96.70(9)
79.46(7)
72.33(8)
105.9(1)
172.03(9)
97.07(9)
81.75(8)
155.70(9)
75.03(7)
–
148.8(2)
101.2(2)
101.8(2)
–
80.0(2)
95.8(2)
98.5(2)
–
72.5(2)
105.4(2)
–
95.4(2)
–
146.77(6)
99.23(8)
104.27(7)
79.32(6)
80.11(6)
99.39(7)
97.06(7)
78.16(6)
71.88(6)
105.20(8)
171.37(8)
92.26(7)
83.37(7)
160.84(7)
79.10(6)
–
146.8(2)
101.0(2)
100.8(2)
–
79.2(2)
99.3(2)
98.6(2)
–
72.4(2)
105.7(3)
–
97.5(2)
–
147.0(1)
99.8(1)
101.8(1)
–
80.3(1)
100. 9(1)
96.3(1)
–
72.2(1)
107.1(1)
–
96.3(1)
–
158.2(2)
–
–
156.2(2)
–
170.8(2)
155.6(1)
–
–

6
S. Pasayat et al. / Polyhedron 67 (2014) 1–10
coordination site around molybdenum(VI) is occupied by a c-pico-
line ligand in complex 1a through its tertiary nitrogen N (3), form-
ing a distorted octahedral complex. The rather large Mo(1)–N(3)
distance [2.429 (5) Å] revealed that the c-picoline moiety is also
2+
rather weakly coordinated to the MoO2 core [16,19,50]. The
2+
Mo–O (oxo) bond distances of the MoO2 group is unexceptional
[16,19,50] and almost equal [1.697 (4)–1.713 (4) Å]. The ligand
2+
coordinates to the MoO2 core in the deprotonated enolate form
because in complexes 1 and 1a the C–O bond distances [C(12)–
O(2)] exhibit values of 1.316 (3) and 1.322 (7) Å respectively and
are closer to a C–O single bond than to a C–O double bond distance.
However, it falls short of the pure C–O single bond distance of
1.42 Å because of the delocalization of electrons.
3.3.3. Description of the X-ray structure of complex 4
We have employed the steric bulk on aroylhydrazones ligand
(H2L4) (Scheme 1) to gain control of the coordination number
around the Mo(VI) center. Structural studies indicate that, with
increasing steric bulk, control of the coordination number around
the Mo(VI) center is achieved by the formation of 5-coordinate
dioxidomolybdenum(VI) complex of the type [MoO2L4] (4)
(Scheme 2). The molecular structure and the atom numbering
scheme of 4 is shown in Fig. 4. In this compound (MoO2L4) the
coordination geometry of Mo(VI) is similar to that observed earlier
[50], which consist of two oxo oxygen atoms, one enolate oxygen,
one phenolate oxygen, and an azomethine nitrogen atom. This is
the rare example of a five-coordinate Mo(VI) complex [50,53]. No
structural evidence for the achievement of hexacoordination
through Mo–Oꢄ ꢄ ꢄMo bridging is observed. The molecular geometry
of MoO2L4 is best represented as a square-pyramid with one axial
oxo oxygen atom [O(3)], and three O atoms [O(1,2,4)] and the N(1)
atom describing the equatorial plane, which is slightly distorted
from an ideal geometry, as reflected in the bond parameters (Ta-
ble 4) around the metal center. The length of the Mo–O(4) bond ly-
ing trans to N(1) is practically equal to Mo–O(3); the position trans
to O(3) remains unoccupied. The other Mo–O and Mo–N distances
are normal, as observed in other structurally characterized
complexes of molybdenum containing these bonds [49,50]. The
bite angles of the ligand at molybdenum, O(2)–Mo(1)–N(1) and
O(1)–Mo(1)–N(1) are 72.2 (1)° and 80.3 (1)°, respectively, generat-
ing five-membered and six-membered chelate rings at the MoVI
center.
Fig. 2. One dimensional polymeric diagram of [MoO2L3], 3. Thermal ellipsoids have
been drawn at 50% probability.
0
2.225–2.233 ÅA, which is comparatively longer than other Mo–N
single bonds. This is due to the trans effect generated by the oxo
group trans to the Mo–N bond [49]. The angular distortion in the
octahedral environment around Mo comes from the bites taken
by the Schiff base ligand, angles O(1)–Mo(1)–N(1) is 80.55 (7)°
and 79.2 (2)° and angles O(2)–Mo(1)–N(1) is 72.33(8)° and 72.4
(2)° for complexes 1 and 3 respectively. For the same reason the
trans angles O(1)–Mo(1)–O(2) and O(4)–Mo(1)–N(1) are signifi-
cantly reduced from the ideal value of 180°. The trans-axial angle
O(3)–Mo(1)–O(5) for complex 1 is 172.03(9)°.
3.3.2. Description of the X-ray structure of complex 1a
The atom numbering scheme of complex 1a is given in Fig. 3
with the relevant bond distances and angles collected in Table 4.
The molecular structure of complex, [MoO2L(c-pic)] has shown
that the dinegative hydrazone ligand (H2L1) binds to the molybde-
num(VI) center by O(1), N(1) and O(2)- donor atoms. The fourth
3.4. Catalytic activity studies
3.4.1. Oxidation of benzoin
The oxidation of benzoin has attracted the attention of
researchers because one of its oxidized products, benzil, is a very
useful intermediate for the synthesis of heterocyclic compounds
Fig. 3. ORTEP diagram of [MoO2L1(
ellipsoids have been drawn at 50% probability.
c
-pic)], 1a with atom labeling scheme. Thermal
Fig. 4. ORTEP diagram of [MoO2L4],
ellipsoids have been drawn at 50% probability.
4 with atom labeling scheme. Thermal

S. Pasayat et al. / Polyhedron 67 (2014) 1–10
7
and benzylic acid rearrangements [54]. The oxidation of benzoin
0.0005 g
0.0010 g
0.0015 g
100
80
60
40
20
0
was successfully achieved with the molybdenum complexes using
30% aqueous H2O2 as oxidant. The products mainly obtained were
benzoic acid, benzaldehyde-dimethylacetal, methylbenzoate and
benzil (Scheme 3).
To optimize the reaction conditions for the maximum oxidation
of benzoin, [MoO2L2(CH3OH)] (2) was considered as a representa-
tive catalyst. The effect of oxidant was studied by considering the
substrate to oxidant ratios of 1:1, 1:2 and 1:3 for the fixed amount
of catalyst (0.0005 g) and substrate (1.06 g, 5 mmol) in 10 mL of
refluxing methanol. As shown in Fig. 5 and entry No. 3 of Table 5,
a maximum of 96% conversion of benzoin was achieved at the sub-
strate to oxidant ratio of 1: 3, in 4 h of reaction time. Lowering the
amount of oxidant decreases the conversion. The effect of amount
of catalyst on the oxidation of benzoin was studied considering
three different amounts of [MoO2L2(CH3OH)] (2) viz. 0.0005,
0.001 and 0.0015 g for the fixed amount of benzoin (1.06 g,
5 mmol) and 30% H2O2 (1.7 g, 15 mmol) in 10 mL of methanol
and the reaction was monitored at reflux temperature of methanol.
A maximum of 96% conversion was achieved with 0.0005 g of
catalyst. This conversion improved only marginally to 98% whereas
0.0015 g of catalyst gave a maximum conversion of 99% in 4 h of
reaction time (Fig. 6). Thus at the expense of catalyst, only
0.0005 g of catalyst can be considered sufficient to optimize other
reaction conditions. The amount of solvent also influences the oxi-
dation of benzoin. It was concluded (Fig. 7 and entry No. 3, 4 and 5
of Table 5) that 10 mL methanol was sufficient to effect maximum
conversion under above optimized reaction conditions. Table 6
summarizes conversion of benzoin under different experimental
conditions. Thus, from these experiments, the best reaction condi-
tions for the maximum oxidation of benzoin as concluded are:
catalyst [MoO2L2(CH3OH)] (2) (0.0005 g), benzoin (1.06 g, 5 mmol),
30% H2O2 (1.7 g, 15 mmol) and refluxing methanol (10 mL).
Fig. 8 presents the selectivity of products along with the conver-
sion of benzoin as a function of time (4 h) under the optimal
experimental conditions as concluded above, i.e. benzoin (1.06 g,
5 mmol), 30% H2O2 (1.7 g, 15 mmol), [MoO2L2(CH3OH)] (2)
(0.0005 g,) and methanol (10 mL) under reflux condition. It is clear
from the plot that all products form with the conversion of ben-
zoin. The highest selectivity of benzoic acid (ca. 51%) was observed
in the first one hour. With the elapse of time its selectivity slowly
decreases and finally becomes almost constant and reaches 47%
after 4 h. Similar results have been observed in case of methyl
benzoate and benzil and reach 23% and 16%, respectively. The
selectivity of benzaldehyde-dimethylacetal increases continuously
from 3% to 14%. Thus, with the maximum benzoin oxidation of 96%
after 4 h of reaction time, the selectivity of the reaction products
varies in the order: benzoic acid (47) > methyl benzoate
(23%), > benzil (16%) > benzaldehyde-dimethylacetal (14%).
0
50
100
Time/min
150
200
250
Fig. 5. Effect of catalyst amount on the oxidation of benzoin. Reaction conditions:
benzoin (1.06 g, 5 mmol), 30% H2O2 (1.7 g, 15 mmol) and methanol (10 mL).
Table 5
Conversion of benzoin (1.06 g, 5 mmol) using [MoO2L2(CH3OH)] (2) as catalyst in 4 h
of reaction time under different reaction conditions.
Entry No.
Catalyst (g)
H2O2 (g mmol)
CH3OH (ml)
Conversion (%)
1
2
3
4
5
6
7
0.0005
0.0005
0.0005
0.0005
0.0005
0.001
0.57, 05
1.14, 10
1.71, 15
1.71, 15
1.71, 15
1.71, 15
1.71, 15
10
10
10
15
20
10
10
83
88
96
89
81
98
99
0.0015
1:1
1:2
1:3
100
80
60
40
20
0
0
50
100
Time/min
150
200
250
Fig. 6. Effect of oxidant amount on the oxidation of benzoin. Reaction conditions:
benzoin (1.06 g, 5 mmol), catalyst amount (0.0005 g) and methanol (10 mL).
Under these reaction conditions other catalysts were also tested
and results are compared in Fig. 9 while Table 6 provides turnover
frequency (TOF) and selectivity details. The data presented in the
table show that other complexes are also catalytically active and
show equally good activity with very high turnover frequency
(TOF: 1021–1301 hꢀ1) but the selectivity order of various products
slightly differs. In the absence of the catalyst, the reaction mixture
showed 60% conversion where selectivity of different products
follows the order: benzil (48%) > benzoic acid (27%) > benzalde-
hyde-dimethylacetal (20%) > methyl benzoate (5%). Thus, these
CH3 CH3
O
O
O
O
d
O
O
Catalyst
OH
OCH3
O
H2O2 / MeOH
OH
a
c
b
Scheme 3. Various oxidation products of benzoin. (a) benzoic acid, (b) benzaldehyde-dimethylacetal, (c) methylbenzoate and (d) benzil.

8
S. Pasayat et al. / Polyhedron 67 (2014) 1–10
10 ml
15 ml
20 ml
100
80
60
40
20
0
100
80
60
40
20
0
[MoO L3]
2
[MoO L2(CH OH)]
2
3
[MoO L1(C2H OH)]
2
5
[MoO L4]
2
[MoO L1(γ-pic)]
2
0
50
100
150
200
250
0
50
100
150
200
250
Time/min
Time/min
Fig. 7. Effect of solvent (methanol) amount on the oxidation of benzoin. Reaction
conditions: benzoin (1.06 g, 5 mmol), catalyst amount (0.0005 g) and 30% H2O2
(1.7 g, 15 mmol).
Fig. 9. Comparison of various catalysts on the oxidation of benzoin. Reaction
conditions: benzoin (1.06 g, 5 mmol), 30% H2O2 (1.7 gm, 15 mmol), catalyst amount
(0.0005 gm) and methanol (10 mL).
Table 6
70% HClO4 (2.86 g, 20 mmol) and water (20 mL). However, the
addition of HClO4 in four equal portions (at t = 0, 45, 90 and
135 min. of reaction time) was necessary to improve the conver-
sion of the substrate and to avoid decomposition of the catalyst.
A total of 3 h was required to complete the reaction. Under the
above conditions, a maximum of 96% conversion was achieved
with [MoO2L2(CH3OH)] (2) and all three products were identified;
Table 7. The presence of three equivalent of H2O2 facilitates not
only the formation of HOBr, which ultimately helps in the oxida-
tive bromination of salicylaldehyde but also affects on the selectiv-
ity of different products. Other catalysts gave similar results.
Lowering the amount of H2O2 increased the formation of 5-bromo-
salicylaldehyde. In the absence of the catalyst, the reaction mixture
gave ca. 40% conversion of salicylaldehyde where selectivity of the
formation of 5-bromosalicylaldehyde is 89%.
Other complexes have also been tested under similar reaction
conditions and the results obtained are summarized in Table 8. It
is clear from the table that all the complexes have equally good
catalytic potential with high turnover frequency. The selectivity
of the 5-bromosalicylaldehyde (56.5–75%) is much high for most
complexes except for [MoO2L4] (4) which exhibits only 47.5%
selectivity. The overall selectivity of three products follows the or-
Effect of different catalysts on the oxidation of benzoin, TOF and product selectivity.
Catalyst (g)
TOF (hꢀ1
)
Conversion (%)
Selectivity (%)a
a
b
c
D
[MoO2L1(C2H5OH)] (1)
1148
1301
1110
1021
1073
96
99
96
95
96
60
29
31
47
40
40
27
22
26
14
25
24
20
31
26
23
22
22
5
18
17
16
13
14
48
[MoO2L1(
c-pic)] (1a)
[MoO2L2(CH3OH)] (2)
[MoO2L3] (3)
[MoO2L4] (4)
Without catalyst
a
(a) Benzoic acid, (b) benzaldehyde-dimethylacetal, (c) methylbenzoate and (d)
benzil.
% Conversion
Benzoic acid
Dimethyl acetal
Methyl benzoate
Benzil
100
80
60
40
20
0
100
80
60
40
20
0
der:
5-bromosalicylaldehyde > 2,4,6-tribromophenol > 3,5-
dibromosalicylaldehyde.
A noteworthy feature of the present catalytic reactions is that,
the recovered solutions of catalysts from the catalytic reaction
mixture were found active up to two cycles with nearly same
conversion (Table S1 and S2). The selectivity of reaction products
of benzoin was also found nearly same but in case of salicylalde-
hyde, the catalyst became more selective towards 5-bromosalicyl-
aldehyde with no formation of tribromo derivative.
50
100
Time/min
150
200
250
Fig. 8. Plot showing percentage conversion of benzoin and the selectivity of benzoic
acid, benzaldehyde dimethylacetal, methyl benzoate and benzil formation as a
function of time. Reaction condition: benzoin (1.06 g, 5 mmol), H2O2 (1.7 g,
15 mmol), [MoO2L2(CH3OH)], 2 (0.0005 g) and 10 mL methanol.
complexes not only enhance the catalytic action, they also alter the
selectivity of the products.
3.4.3. Reactivity of complexes with H2O2
Dioxidomolybdenum (VI) complexes are known to react with
H2O2 to form the corresponding oxidoperoxido complexes. In order
to throw some light on the reaction mechanism of catalytic reac-
tions, complex [MoO2L2(CH3OH)] (2) dissolved in DMSO was
reacted with H2O2 and spectral changes were monitored by elec-
tronic absorption spectroscopy. Thus, the stepwise additions of
H2O2 (1.47 g, 13 mmol of 30% H2O2 dissolved in 5 mL of DMSO)
to 25 mL of ca. 6.7 ꢁ 10ꢀ3 M solution of [MoO2L2(CH3OH)] (2) in
DMSO causes the decrease in the intensities of the 468 nm band
and finally disappear (Fig. 10). Simultaneously, two bands start
appearing at 369 and 383 nm and become intense after addition
of excess of H2O2. The intensity of the 337 nm band decreases
slowly along with the appearance of a shoulder band at ca.
3.4.2. Oxidative bromination of salicylaldehyde
Oxidative bromination of salicylaldehyde, a functional mimic of
haloperoxidases, in aqueous solution at room temperature has also
been carried out successfully. By using these dioxidomolybde-
num(VI) complexes as catalyst precursors in the presence of KBr,
HClO4 and H2O2 gave mainly three products, namely 5-bromosali-
cylaldehyde, 3,5-dibromosalicylaldehyde and 2,4,6-tribromophe-
nol; (Scheme 4). After several trials (Table 7), the best suited
reaction conditions obtained for the maximum conversion of
5 mmol (0.610 g) of salicylaldehyde were: KBr (1.785 g, 15 mmol),
aqueous 30% H2O2 (1.71 g, 15 mmol), catalyst (0.0005), aqueous

S. Pasayat et al. / Polyhedron 67 (2014) 1–10
9
OH
OH
OH
OH
Br
Catalyst
Br
Br
O
O
O
KBr/H2O2/HClO4
Br
Br
Br
c
a
b
Scheme 4. Main products obtained upon oxidative bromination of salicylaldehyde. (a) 5-bromosalicylaldehyde, (b) 3,5-dibromosalicylaldehyde and (c) 2,4,6-
tribromophenol.
Table 7
Results of oxidative bromination of salicylaldehyde catalyzed by [MoO2L2(CH3OH)] (2) after 3 h of contact time.
Entry No.
Catalyst (g)
H2O2 (g mmol)
KBr (g mmol)
HClO4 (g mmol)
Conversion (%)
1
2
3
4
5
6
7
8
9
0.0005
0.0005
0.0005
0.0005
0.0005
0.0005
0.0005
0.0010
0.0015
0.57, 5
1.78, 15
1.78, 15
1.78, 15
0.59, 5
1.19, 10
1.78, 15
1.78, 15
1.78, 15
1.78, 15
2.86, 20
2.86, 20
2.86, 20
2.86, 20
2.86, 20
1.43, 10
2.14, 15
2.86, 20
2.86, 20
66
81
96
57
77
67
89
98
99
1.14, 10
1.71, 15
1.71, 15
1.71, 15
1.71, 15
1.71, 15
1.71, 15
1.71, 15
Table 8
Effect of different catalysts on the oxidative bromination of salicylaldehyde, TOF and product selectivity.
Catalyst (g)
TOF (hꢀ1
)
Conversion (%)
Selectivity (%)a
Monobromo
Dibromo
Tribromo
[MoO2L1(C2H5OH)] (1)
1555
1673
1482
1415
1470
98
95
96
99
99
40
60.6
75
75
56.6
47.5
89
1.6
4.4
12.1
1.7
1.6
0.2
37.8
20.5
12.6
41.7
51.0
10.8
[MoO2L1(
c-pic)] (1a)
[MoO2L2(CH3OH)] (2)
[MoO2L3] (3)
[MoO2L4] (4)
Without catalyst
a
(a) 5-Bromosalicylaldehyde, (b) 3,5-dibromosalicylaldehyde and (c) 2,4,6-tribromophenol.
a bromide ion to give bromine equivalent intermediate(s). Such an
intermediate may then brominate an appropriate organic substrate
[55].
312 nm
337 nm
369 nm
383 nm
1.2
1.0
0.8
0.6
0.4
0.2
0.0
468 nm
4. Conclusions
Synthesis of four new dioxidomolybdenum(VI) complexes with
aroylhydrazone of naphthol-derivative (1, 2, 3 and 4) and one
mixed-ligand mononuclear complex (1a) have been prepared and
characterized. The one labile binding site in these complexes has
an added advantage as has been shown by reacting complex 2 with
H2O2 to give corresponding peroxido species without changing ori-
ginal coordinating sites i.e. ONO coordination of ligands. Again the
labile peroxido group of complexes has an added advantage as they
transfer oxygen in catalytic oxidation as has been demonstrated for
the oxidation by peroxide, of benzoin using these complexes as
catalysts. Under optimized reaction conditions benzoin gives 95–
99% conversion with four reaction products benzoic acid, benzal-
dehyde-dimethylacetal, methylbenzoate and benzil. The functional
mimic of haloperoxidase is demonstrated by the oxidative bromin-
ation of salicylaldehyde where these complexes have shown 95–
99% conversion of salicylaldehyde to brominated and other
products with high turnover frequency.
300
400
500
600
Wavelength/nm
Fig. 10. UV–Vis spectral changes observed during titration of [MoO2L2(CH3OH)], 2
with H2O2. The spectra were recorded after successive additions of 1-drop portions
of 30% H2O2 (13 mmol) dissolved in 5 mL of DMSO to 25 mL of 6.7 ꢁ 10ꢀ3
M
solution in DMSO.
320 nm. We have interpreted this result in terms of the formation
of the oxidoperoxidomolybdenum(VI) complex. The new bands at
383 and 368 nm may be assigned to LMCT and n–p⁄ transitions,
respectively. The latter one was not visible in [MoO2L2(CH3OH)]
(2) while the band at 320 nm is assignable to
p–
p⁄ transition.
The oxidoperoxidomolybdenum(VI) complex finally transfers oxy-
gen to the benzoin to give the various oxidation products. The
intermediate peroxido complex is likely to form hydroperoxido-
molybdenum(VI) complex also in the presence of H+ that oxidizes
Declaration
I do also hereby declare that the work described in the submis-
sion is new and has not been published previously. It is not under

10
S. Pasayat et al. / Polyhedron 67 (2014) 1–10
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consideration for publication elsewhere. Its publication is
approved by all authors and explicitly by the responsible authori-
ties where the work was carried out.
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2008. S. Pasayat and Y. P. Patil gratefully acknowledge the Council
of Scientific and Industrial Research, New Delhi, India, for the
award of research fellowship.
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