Cis-Dioxomolybdenum(VI) complexes with unsymmetric linear tetradentate ligands: Syntheses, structures and bromoperoxidase activities

Article abstract of DOI:10.1002/aoc.3403

Reactions of [MoO₂(acetylacetonate)₂], 2-((2-(2-hydroxyethylamino)ethylamino)methyl)-4-R-phenols (H₂Lⁿ, n = 1-5 for R = H, Me, OMe, Cl and Br, respectively) and KOH in 1:1:2 mole ratio in methanol afford a series of complexes having the general formula cis-[MoO₂(Lⁿ)] (1, 2, 3, 4, 5) in 81-86% yields. The complexes have been characterized using elemental analysis, spectroscopy (infrared, UV-visible, and ¹H NMR, ¹³C NMR and ¹³C-DEPT NMR) and electrochemical measurements. The molecular structures of 1, 2, 3, 4 have been determined using single-crystal X-ray crystallography. In each of 1, 2, 3, 4, the ONNO-donor 6,5,5-membered fused chelate rings forming (Lⁿ)^2- and the two mutually cis oxo groups assemble a distorted octahedral N₂O₄ coordination sphere around the metal centre. In the crystal lattice, each of 1, 2, 3, 4 forms a one-dimensional infinite chain structure via intermolecular N-H...O hydrogen bonding interactions. In cyclic voltammograms, the diamagnetic complexes display an irreversible metal-centred reduction in the potential range -0.73 to -0.88 V (vs Ag/AgCl). The physicochemical data are consistent with a very similar gross molecular structure for all of 1, 2, 3, 4, 5. All the complexes exhibit decent bromoperoxidase activities and are also able to effectively catalyse benzoin and methyl(phenyl)sulfide oxidation reactions.

Full text of DOI:10.1002/aoc.3403

Full paper
Received: 12 August 2015
Revised: 24 September 2015
Accepted: 8 October 2015
Published online in Wiley Online Library: 21 December 2015
(wileyonlinelibrary.com) DOI 10.1002/aoc.3403
cis-Dioxomolybdenum(VI) complexes with
unsymmetric linear tetradentate ligands:
syntheses, structures and bromoperoxidase
activities
Sathish Kumar Kurapati and Samudranil Pal*
n
Reactions of [MoO
2
(acetylacetonate)
2
], 2-((2-(2-hydroxyethylamino)ethylamino)methyl)-4-R-phenols (H
2
L , n = 1–5 for R = H, Me,
OMe, Cl and Br, respectively) and KOH in 1:1:2 mole ratio in methanol afford a series of complexes having the general formula
n
cis-[MoO (L )] (1–5) in 81–86% yields. The complexes have been characterized using elemental analysis, spectroscopy (infrared,
2
1
13
13
UV–visible, and H NMR, C NMR and C-DEPT NMR) and electrochemical measurements. The molecular structures of 1–4 have
been determined using single-crystal X-ray crystallography. In each of 1–4, the ONNO-donor 6,5,5-membered fused chelate rings
n 2ꢀ
2 4
forming (L ) and the two mutually cis oxo groups assemble a distorted octahedral N O coordination sphere around the metal
centre. In the crystal lattice, each of 1–4 forms a one-dimensional infinite chain structure via intermolecular N–HꢁꢁꢁO hydrogen
bonding interactions. In cyclic voltammograms, the diamagnetic complexes display an irreversible metal-centred reduction in
the potential range ꢀ0.73 to ꢀ0.88 V (vs Ag/AgCl). The physicochemical data are consistent with a very similar gross molecular
structure for all of 1–5. All the complexes exhibit decent bromoperoxidase activities and are also able to effectively catalyse
benzoin and methyl(phenyl)sulfide oxidation reactions. Copyright © 2015 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: molybdenum(VI) complexes; unsymmetric ONNO-donor; crystal structures; physicochemical properties; catalytic applications
In the present work, we synthesized a new series of cis-
Introduction
dioxomolybdenum(VI) complexes with the unsymmetric linear
Haloperoxidases are capable of efficiently catalysing the halogenation
tetradentate ONNO-donor 2-((2-(2-hydroxyethylamino)ethylamino)-
of a variety of organic compounds in the presence of hydrogen perox-
ide and halides (Cl , Br and I ).
compounds, brominated ones are of particular interest due to their
extensive applications in the synthesis of flame retardants, phar-
maceuticals, biocides and pesticides.
of its convenient and eco-friendly nature, oxidative bromination with
bromoperoxidases and their mimics has drawn considerable atten-
tion over traditional bromination using Br
based bromoperoxidases are most widespread among all the
naturally occurring bromoperoxidases.
methyl)-4-R-phenol (R= H, Me, OMe, Cl and Br) (Scheme 1) and
ꢀ
ꢀ
ꢀ [1–14]
Among halogenated organic
evaluated their catalytic efficiencies in oxidative bromination
reactions as well as in benzoin and methyl(phenyl)sulfide oxidation
reactions. Herein, we describe the details of syntheses, characteriza-
tion, X-ray structures and catalytic behaviours of these complexes.
[15–17]
In recent times, because
[
6–14,18–21]
2
.
Vanadium(V)-
Experimental
[
6–14]
As a result, complexes
Materials and methods
of oxovanadium(V) have been studied extensively as structural
[7,22–25]
[MoO
2 2
(acac) ] (acac = acetylacetonate) was prepared according to a
and functional models for bromoperoxidases.
In comparison,
[39]
literature procedure. The hydrochloride salts of 2-((2-(2-hydroxy-
ethylamino)ethylamino)methyl)-4-R-phenols (H
reports on bromoperoxidase activities of cis-dioxomolybdenum(VI)
n
[
26–31]
2
L ꢁ2HCl) were pre-
complexes are rather limited,
to efficiently catalyse various organic oxidation reactions.
These catalytically active complexes mostly have bi- and tridentate
even though they are known
[40]
[32–37]
pared following a reported procedure. All other chemicals and
solvents used in this work were of reagent grade and used as
received without further purification.
Elemental (CHN) analysis data were obtained using a Thermo
Finnigan Flash EA1112 series elemental analyser. Magnetic suscep-
tibility measurements were performed with a Sherwood Scientific
[26–37]
ligands rather than tetradentate ligands.
Recently, we have
reported a series of cis-dioxomolybdenum(VI) complexes with an
unsymmetric tripodal tetradentate NO -donor ligand system and
3
evaluated their catalytic efficiencies in oxidative bromination and
[38]
benzoin oxidation reactions. These complexes provide improved
product selectivity but inferior turnover frequencies (TOFs) in com-
parison to the previously reported analogous reactions catalysed
by cis-dioxomolybdenum(VI) complexes with bi- and tridentate
*
Correspondence to: Samudranil Pal, School of Chemistry, University of Hyderabad,
Hyderabad 500046, India. E-mail: spal@uohyd.ac.in
[29–31]
School of Chemistry, University of Hyderabad, Hyderabad 500046, India
ligands.
Appl. Organometal. Chem. 2016, 30, 116–124
Copyright © 2015 John Wiley & Sons, Ltd.

2
+
cis-{MoO2} complexes and their bromoperoxidase activities
3
cis-[MoO
2 2 5
(L )] (3). Yield 86%. Anal. Calcd (%) for C12H18MoN O
(
7
366.22): C, 39.36; H, 4.95; N, 7.65. Found (%): C, 39.25; H, 4.91; N,
ꢀ
1
.58. Selected IR bands (cm ): 3210 and 3150 (νN–H), 2940–2832
3
ꢀ1
2
(ν
C–H), 915 and 887 ðνMoO Þ. UV–visible (λmax, nm (ε, 10 M
2
ꢀ
1
1
cm )): 340 (4.5), 301 (6.5). H NMR (δ, ppm): 6.70 (d, 6 Hz, 1H, H ),
6
N H), 4.56 (d, 16Hz, 1H, H ), 4.48 (m, 1H, H ), 4.38 (m, 1H, H ),
3
5
3
1
.69 (s, 1H, H ), 6.58 (d, 8 Hz, 1H, H ), 5.73 (s, 1H, N H), 5.32 (s, 1H,
2
7a
11a
11b
7
b
8a
.74 (d, 16 Hz, 1H, H ), 3.68 (s, 3H, 4-OMe), 3.54 (m, 1H, H ), 2.95
8b
9a,b
10a,b 13
(
m, 1H, H ), 2.60 (m, 2H, H ), 2.41 (m, 2H, H
). C NMR
4
1
6
2
(δ, ppm): 155.13 (C ), 153.07 (C ), 124.44 (C ), 119.92 (C ),
5
3
11
7
1
5
15.56 (C ), 114.57 (C ), 73.51 (C ), 56.23 (4-OMe), 53.88 (C ),
1
0
8
9
1.98 (C ), 47.63 (C ), 47.27 (C ). Epc (V): ꢀ0.80.
n
Scheme 1. Synthesis of cis-[MoO
2
(L )] (1–5 for n = 1–5, respectively) from
4
cis-[MoO
2 2 4
(L )] (4). Yield 81%. Anal. Calcd (%) for C11H15ClMoN O
the corresponding dihydrochloride salts of unsymmetric linear tetradentate
n
(370.64): C, 35.65; H, 4.08; N, 7.56. Found (%): C, 35.48; H, 4.13; N,
7.51. Selected IR data (cm ): 3205 and 3140 (νN–H), 2960–2850
2
-((2-(2-hydroxyethylamino)ethylamino)methyl)-4-R-phenols (H
2
L , n = 1–5
ꢀ1
for R = H, Me, OMe, Cl and Br, respectively).
3
ꢀ1
(
ν
), 931 and 887 ðνMoO Þ. UV–visible (λ , nm (ε, 10 M
C–H
ꢀ
2
1
max
1
5
balance. A Shimadzu LCMS 2010 liquid chromatograph mass spec-
trometer was used for verification of compound purity. Solution
electrical conductivities were measured using a Digisun DI-909
conductivity meter. Infrared spectra on KBr pellets were collected
with a Thermo Scientific Nicolet 380 FT-IR spectrophotometer. A
Shimadzu UV-3600 UV-VIS-NIR spectrophotometer was used to
cm )): 314 (5.4), 287 (7.1). H NMR (δ, ppm): 7.16 (s, 1H, H ), 7.13
d, 12 Hz, 1H, H ), 6.65 (d, 12Hz, 1H, H ), 5.80 (s, 1H, N H), 5.48 (s,
H, N H), 4.52 (d, 16Hz, 1H, H ), 4.50 (m, 1H, H ), 4.41 (m, 1H,
3
2
1
(
1
H
2
(
(
4
2
7a
11a
1
1b
7b
8a
8b
), 3.80 (d, 16Hz, 1H, H ), 3.59 (m, 1H, H ), 2.96 (m, 1H, H ),
9a
9b
10a,b 13
.68 (m, 1H, H ), 2.60 (m, 1H, H ), 2.41 (m, 2H, H
). C NMR
1
5
3
4
δ, ppm): 160.21 (C ), 130.07 (C ), 128.94 (C ), 125.95 (C ), 123.07
6 2 11 7 10 8
1
13
record the electronic spectra. H NMR (400 MHz) and C NMR
100 MHz) spectra were recorded with a Bruker NMR spectrometer
C ), 121.31 (C ), 73.87 (C ), 53.28 (C ), 52.14 (C ), 47.55 (C ),
9
(
7.29 (C ). E (V): ꢀ0.83.
pc
5
using tetramethylsilane as internal standard. A CH Instruments
model 620A electrochemical analyser was used for cyclic volta-
mmetric experiments. GC measurements were performed with a
Shimadzu GCMS-QP2010 instrument.
cis-[MoO (L )] (5). Yield 82%. Anal. Calcd (%) for C H BrMoN O
2
11 15
2
4
(
415.09): C, 31.83; H, 3.64; N, 6.75. Found (%): C, 31.96; H, 3.58; N,
ꢀ
1
6
9
3
.83. Selected IR bands (cm ): 3205 (νN–H), 2925–2843 (ν ),
C–H
ꢀ1
3
ꢀ1
5
14 and 876 ðνMoO Þ. UV–visible (λ , nm (ε, 10 M cm )):
2
1
max
17 (5.6), 285 (7.5). H NMR (δ, ppm): 7.30 (s, 1H, H ), 7.27 (d,
3
2
1
n
8 Hz, 1H, H ), 6.62 (d, 8 Hz, 1H, H ), 5.83 (s, 1H, N H), 5.50 (s, 1H,
N H), 4.54 (d, 16 Hz, 1H, H ), 4.51 (m, 1H, H ), 4.41 (m, 1H,
H
General procedure for synthesis of cis-[MoO (L )] (1–5)
2
2
7a
11a
11b
7b
8a
8b
Solid [MoO (acac) ] (0.3 mmol) was added to a solution of the cor-
), 3.80 (d, 16Hz, 1H, H ), 3.59 (m, 1H, H ), 2.96 (m, 1H, H ),
2
2
n
9a
9b
10a,b 13
responding H L ꢁ2HCl (0.3 mmol) and KOH (0.6 mmol) in methanol
2.69 (m, 1H, H ), 2.60 (m, 1H, H ), 2.41 (m, 2H, H
). C NMR
2
1
5
3
6
(
25 ml). The resulting bright yellow reaction mixture was refluxed
(δ, ppm): 160.65 (C ), 132.89 (C ), 131.82 (C ), 126.55 (C ), 121.82
2 4 11 7 10 8
for 0.5 h and then cooled to room temperature under ambient
conditions. The pale yellow to bright yellow microcrystalline solid
separated was collected by filtration and washed with water,
methanol and diethyl ether, and finally dried in air.
(C ), 110.73 (C ), 73.87 (C ), 53.17 (C ), 52.12 (C ), 47.53 (C ),
9
47.27 (C ). E (V): ꢀ0.88.
pc
Catalysis measurements
1
cis-[MoO (L )] (1). Yield 83%. Anal. Calcd (%) for C H MoN O
2
11 16
2
4
Catalytic activities of 1–5 in oxidative bromination of various olefinic
compounds and in oxidation of benzoin and methyl(phenyl)sulfide
were examined. The details of optimized reaction conditions are
described below.
(
8
336.20): C, 39.30; H, 4.80; N, 8.33. Found (%): C, 39.46; H, 4.71; N,
ꢀ
1
.45. Selected IR bands (cm ): 3238 and 3155 (νN–H), 2964–2865
3
ꢀ1
(ν
C–H), 925 and 876 ðνMoO Þ. UV–visible (λmax, nm (ε, 10 M
2
1
ꢀ
1
cm )): 313 (4.6), 273 (6.5). H NMR (δ, ppm): 7.12 (dd, 8, 8 Hz, 1H,
H ), 7.08 (d, 8 Hz, 1H, H ), 6.74 (dd, 8, 8 Hz, 1H, H ), 6.65 (d, 8 Hz,
1
4
(
2
1
3
5
4
Oxidative bromination
2
1
2
7a
H, H ), 5.77 (s, 1H, N H), 5.39 (s, 1H, N H), 4.59 (d, 16 Hz, 1H, H ),
1
1a
11b
7b
.51 (m, 1H, H ), 4.41 (m, 1H, H ), 3.78 (d, 16 Hz, 1H, H ), 3.57
m, 1H, H ), 2.96 (m, 1H, H ), 2.66 (m, 1H, H ), 2.59 (m, 1H, H ),
.40 (m, 2H, H
Amounts of 10 mmol of substrate, 15 mmol of H
w/w aqueous solution), 20 mmol of KBr, 20 mmol of HClO
of 60% aqueous HClO solution) and 0.01 mmol of the catalyst
(L )] were taken in 6 ml of the mixed solvent water–
2
O
2
(1.8 ml of 30%
8
a
8b
9a
9b
(2.4 ml
4
1
0a,b 13
1
5
). C NMR (δ, ppm): 161.15 (C ), 130.66 (C ),
4
3
4
6
2
11
n
29.34 (C ), 123.89 (C ), 120.08 (C ), 119.58 (C ), 73.74 (C ), 53.83
cis-[MoO
2
7
10
8
9
(C ), 52.05 (C ), 47.59 (C ), 47.26 (C ). Epc (V): ꢀ0.79.
methanol (1:1). The resulting mixture was stirred at room tempe-
rature for 0.25 to 2.5h (substrate dependent). Then the reaction
mixture was diluted with 50 ml of water and extracted with
dichloromethane (2× 10 ml). The dichloromethane extracts were
combined, dried over anhydrous sodium sulfate and finally diluted
to 40 ml with dry dichloromethane. This diluted solution was
analysed using GC-MS for identification and yield determination
(from the areas under the peaks) of the products.
2
cis-[MoO
2 2 4
(L )] (2). Yield 85%. Anal. Calcd (%) for C12H18MoN O
(350.22): C, 41.15; H, 5.18; N, 8.00. Found (%): C, 41.34; H, 5.09; N,
ꢀ
1
8
.12. Selected IR bands (cm ): 3205 and 3150 (νN–H), 2930–2840
3
ꢀ1
3
(ν
C–H), 926 and 882 ðνMoO Þ. UV–visible (λmax, nm (ε, 10 M
2
ꢀ
1
1
cm )): 324 (4.0), 275 (5.2). H NMR (δ, ppm): 6.94 (d, 8 Hz, 1H, H ),
6
N H), 4.58 (d, 16 Hz, 1H, H ), 4.49 (m, 1H, H ), 4.38 (m, 1H, H ),
3
5
2
1
.90 (s, 1H, H ), 6.57 (d, 8 Hz, 1H, H ), 5.77 (s, 1H, N H), 5.35 (s, 1H,
2
7a
11a
11b
7
b
8a
8b
.74 (d, 16 Hz, 1H, H ), 3.55 (m, 1H, H ), 2.95 (m, 1H, H ), 2.60
Benzoin oxidation
9
a,b
10a,b
5
13
(
(
(
m, 2H, H ), 2.41 (m, 2H, H
), 2.22 (s, 3H, 4-Me). C NMR
1
3
4
δ, ppm): 158.95 (C ), 130.98 (C ), 129.77 (C ), 128.67 (C ), 123.52
C ), 119.28 (C ), 73.67 (C ), 53.84 (C ), 51.98 (C ), 47.65 (C ),
Amounts of 20 mmol of H
and 0.01 mmol of the catalyst cis-[MoO
anol (6 ml) solution of 10mmol of benzoin. The reaction mixture
2 2
O (2.4ml of 30% w/w aqueous solution)
6
2
11
7
10
8
n
2
(L )] were added to a meth-
9
4
7.24 (C ), 21.05 (4-Me). Epc (V): ꢀ0.73.
Appl. Organometal. Chem. 2016, 30, 116–124
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

S. K. Kurapati and S. Pal
[
43]
was stirred at room temperature for 1 h, diluted with 40 ml of water
and finally extracted with dichloromethane (2× 10 ml). The com-
bined dichloromethane extracts were dried over anhydrous sodium
sulfate, diluted to 40 ml with dry dichloromethane and subjected to
GC-MS analysis to identify the products and determine (using the
peak areas) their yields.
radiation (λ = 0.71073 Å). The CrysAlisPro package was used for
data collection, reduction and absorption correction. The structure
of each of 1–4 was solved by direct method and refined on F by
full-matrix least-squares procedures. All non-hydrogen atoms were
refined anisotropically, while all hydrogen atoms were included in
the structure factor calculation using a riding model. Structure so-
lution and refinement were done using the SHELX-97 programs
All the molecular graphic
figures were made using either the Platon
2
[44]
Methyl(phenyl)sulfide oxidation
[45]
as provided in the WinGX package.
[46]
[47]
or the Mercury
Amounts of 10 mmol of methyl(phenyl)sulfide, 15 mmol of H
2 2
O
packages. Selected crystal and refinement data for 1–4 are listed
in Table 1.
(
1.2 ml of 30% w/w aqueous solution) and 0.01 mmol of the catalyst
n
2
cis-[MoO (L )] were stirred at room temperature for 1 h. The reac-
tion mixture was then extracted with dichloromethane (2× 10 ml).
After drying over anhydrous sodium sulfate, the combined dichlo-
romethane extracts were diluted to 40ml with dry dichlorometh-
ane and analysed using GC-MS for identification of products and
determination of their yields from the peak areas.
Results and discussion
Synthesis and some properties
Complexes 1–5 were synthesized by reacting one mole equiva-
n
lent each of [MoO
2
(acac)
2
] and the corresponding H
2
L ꢁ2HCl in
X-ray crystallography
the presence of two mole equivalents of KOH in refluxing metha-
nol. They were obtained as air-stable yellow microcrystalline
materials in 81–86% yields. The elemental (CHN) analysis data
are in good agreement with those calculated for the molecular
formulas of 1–5. The diamagnetic nature of 1–5 clearly establishes
the +6 oxidation state of the molybdenum centre in each com-
plex. All the complexes are highly soluble in dimethylsulfoxide
and dimethylformamide, sparingly soluble in methanol, ethanol
and acetonitrile, and insoluble in dichloromethane, chloroform,
toluene and hexane. In solutions, 1–5 behave as non-electrolytes.
Single crystals of 1–4 were obtained by heating a mixture of [MoO2
n
(
acac) ] (0.05 mmol), the corresponding H L ꢁ2HCl (0.05 mmol) and
2
2
KOH (0.1 mmol) in 25 ml of methanol on a digital water bath
ꢀ
1
at 90 °C for 10 min and then uniformly cooling (by ca 30 °C h ) the
reaction mixture to room temperature in about 2 h. We tried to grow
X-ray-quality crystals of 5 using various crystallization techniques, but
none of them was successful. Determination of the unit cell para-
meters and the collection of the intensity data at 298 K for each of
1
and 3 were performed with a Bruker-Nonius SMART APEX CCD
single crystal diffractometer using graphite-monochromated Mo Kα
[41]
radiation (λ = 0.71073 Å). The SMART and the SAINT-Plus packages
Crystal structures
were used for data acquisition and data extraction, respectively. Ab-
[42]
sorption correction was done using the SADABS program. The
Complex 1 crystallizes in the non-centrosymmetric orthorhombic
unit cell parameters and the intensity data at 298 K for both 2 and
1
space group Pca2 , whereas complexes 2–4 crystallize in the cen-
4
were obtained with an Oxford Diffraction Xcalibur Gemini single
trosymmetric triclinic space group P1. In each structure, the asym-
crystal X-ray diffractometer using graphite-monochromated Mo Kα
metric unit is comprised of a single complex molecule. Figure 1
Table 1. Selected crystal data and refinement summary for 1–4
1
2
3
4
Chemical formula
Formula weight
Crystal system
Space group
a (Å)
C
11
H
16MoN
336.20
Orthorhombic
2
O
4
C
12
H
18MoN
2
O
4
C
12
H
18MoN
2
O
5
11 2 5
C H15ClMoN O
350.22
Triclinic
P1
366.22
Triclinic
P1
370.64
Triclinic
P1
Pca2
10.176(5)
6.880(4)
17.762(9)
90
1
7.1224(11)
7.1472(6)
10.1444(9)
10.6134(9)
101.026(1)
108.748(1)
98.481(1)
696.93(10)
2
7.0918(6)
10.0620(7)
10.3407(8)
106.480(7)
104.952(7)
98.345(6)
664.36(9)
2
b (Å)
10.1493(14)
10.3743(17)
106.585(13)
104.050(13)
100.195(12)
672.32(16)
2
c (Å)
α (°)
β (°)
90
γ (°)
V (Å )
90
3
1243.5(11)
4
Z
ꢀ
3
ρ (g cm
μ (mm
)
1.796
1.730
1.745
1.853
ꢀ1
)
1.063
0.987
0.962
1.199
Reflections collected
Reflections unique
Reflections [I ≥ 2σ(I)]
Parameters
11 787
2435
4574
7307
4557
2744
2731
2443
2392
2377
2445
1919
163
173
181
172
R1, wR2 [I ≥ 2σ(I)]
R1, wR2 [all data]
0.0178, 0.0439
0.0181, 0.0443
1.165
0.0408, 0.0873
0.0496, 0.0918
1.071
0.0316, 0.0729
0.0369, 0.0759
1.045
0.0525, 0.1315
0.0682, 0.1400
1.036
2
GOF on F
ꢀ
3
)
Δρmax, Δρmin (e Å
0.405/ꢀ0.682
0.772/ꢀ0.585
0.460/ꢀ0.378
1.750/–0.543
wileyonlinelibrary.com/journal/aoc
Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2016, 30, 116–124

2
+
cis-{MoO2} complexes and their bromoperoxidase activities
oxo groups form a distorted octahedral N
metry around the molybdenum. In all of 1–4, the Mo–N(amine)
Mo–N(1) (2.323(5)–2.365(2) Å) and Mo–N(2) (2.318(2)–2.353(3) Å))
bond lengths are significantly longer than the Mo–O(phenolate)
2 4
O coordination geo-
(
(
(
Mo–O(1) (1.939(2)–1.9816(17) Å)) and Mo–O(ethanolate) (Mo–O
2) (1.942(3)–1.946(4) Å)) bond lengths. This lengthening of the
Mo–N bonds is due to the strong trans effects imposed by the
oxo groups (O(3) and O(4)) on the secondary amine-N atoms (N(1)
and N(2)). It is worth noting that the Mo–O(ethanolate) bond
lengths are very similar but the Mo–O(phenolate) bond lengths
vary significantly with the change of the substituent (R) at para
n 2ꢀ
position of the phenolate fragment of (L ) (Table 2). Although
the variation of the Mo–O(phenolate) bond length does not re-
flect a systematic trend with the change of the electronic nature
of R, for the electron-releasing groups (R = Me and OMe) the
bond lengths are shorter than for R = H or electron-withdrawing
Cl. The bond angles in the five-membered chelate rings are in
the range 72.63(7)–74.52(7)° and those in the six-membered
chelate rings are in the range 76.30(7)–81.25(11)°. In contrast,
the cis-MoO bond angles (108.75(12)–109.1(2)°) are very similar
2
and significantly larger. The remaining cis bond angles span the
broad range of 82.01(9)–103.51(15)°, while the trans bond angles
are within a relatively narrow range of 155.51(10)–163.82(10)°. In
general, the bond lengths and bond angles within the metal
ion coordination spheres of 1–4 (Table 2) lie in the range re-
ported for cis-dioxomolybdenum(VI) complexes with ligands
1
2
Figure 1. ORTEPs of cis-[MoO
In both plots, thermal ellipsoids for all non-hydrogen atoms are shown at the
0% probability level.
2 2
(L )] (1) (top) and cis-[MoO (L )] (2) (bottom).
3
[
29–31,38,48–62]
coordinating through comparable donor atoms.
It may be noted that metal coordination to the secondary amine-
N atoms (N(1) and N(2)) of (L ) makes them asymmetric centres
and hence molecules of all the complexes are chiral. However,
crystallizations of 2–4 in the centrosymmetric space group P1
clearly suggest that in each case, the crystals as well as the bulk
sample are racemic in nature. In view of the fact that 1 crystallizes
illustrates the molecular structures of 1 and 2, while Fig. S1
supporting information) shows the molecular structures of 3 and 4.
The bond parameters related to the metal centres are listed in Table 2.
n 2ꢀ
(
On the whole, the molecular structures of 1–4 are very similar. In each
n 2ꢀ
complex molecule, the ligand (L ) coordinates to the metal centre
through a phenolate-O, two secondary amine-N and an ethanolate-O
atoms and forms 6,5,5-membered fused chelate rings. The
1
in the non-centrosymmetric space group Pca2 , we recorded the
n 2ꢀ
solid-state circular dichroism (CD) spectrum of its bulk form. No
signal in the CD spectrum of 1 indicates that like 2–4 here also
the bulk sample contains both the enantiomers in 1:1 ratio and
hence it is optically inactive. Thus there is a spontaneous resolution
of 1 during the course of its crystallization.
unsymmetric ONNO-donor (L ) and the two mutually cis-oriented
Table 2. Metal-centred bond lengths (Å) and angles (°) for 1–4
1
2
3
4
The presence of two secondary amine functionalities in each of
1–4 has led us to search for the intermolecular conventional hydro-
gen bonding-assisted self-assembled supramolecular structures, if
any. Indeed, both N–H functionalities in all of 1–4 are involved in
intermolecular hydrogen bonding with metal-bound O atoms. Both
the phenolate-O and the ethanolate-O atoms (O(1) and O(2),
respectively) in 1 and the ethanolate-O and one oxo group (O(2)
and O(3), respectively) in each of 2–4 act as acceptors in these
N–HꢁꢁꢁO interactions. The geometrical parameters for all three
types of N–HꢁꢁꢁO hydrogen bonds are summarized in Table 3.
Self-assembly of the molecules of 1–4 via these interactions leads
to one-dimensional chain structures in the corresponding crystal
lattices. The self-assembled structures of 1 and 2 are depicted in
Fig. 2, while those of 3 and 4 are shown in Fig. S2 (supporting
information). In the case of 1, the one-dimensional chain formed
by the N(1)–HꢁꢁꢁO(1) and N(2)–HꢁꢁꢁO(2) interactions propagates
along the a-axis of the non-centrosymmetric lattice. In contrast,
for 2–4 the one-dimensional chain structures lie along the b-axis
of the centrosymmetric lattices. In the chain, each enantiomer is
flanked by two opposite enantiomers. One of the two opposite
enantiomers is connected by a pair of reciprocal N(1)–HꢁꢁꢁO(2)
interactions, while the other one is connected by a pair of reciprocal
N(2)–HꢁꢁꢁO(3) interactions.
Mo–O(1)
1.9816(17)
1.9451(15)
1.7063(16)
1.7134(16)
2.365(2)
2.318(2)
156.09(7)
93.62(8)
99.68(9)
76.30(7)
86.54(7)
101.22(8)
93.19(8)
84.08(7)
74.52(7)
108.84(9)
161.53(7)
91.58(8)
88.32(8)
158.07(7)
72.63(7)
1.939(3)
1.942(3)
1.939(2)
1.942(2)
1.946(5)
1.946(4)
1.709(4)
1.691(4)
2.323(5)
2.353(5)
155.8(2)
93.6(2)
Mo–O(2)
Mo–O(3)
1.704(3)
1.712(2)
Mo–O(4)
1.690(3)
1.708(2)
Mo–N(1)
2.329(3)
2.334(2)
Mo–N(2)
2.353(3)
2.336(3)
O(1)–Mo–O(2)
O(1)–Mo–O(3)
O(1)–Mo–O(4)
O(1)–Mo–N(1)
O(1)–Mo–N(2)
O(2)–Mo–O(3)
O(2)–Mo–O(4)
O(2)–Mo–N(1)
O(2)–Mo–N(2)
O(3)–Mo–O(4)
O(3)–Mo–N(1)
O(3)–Mo–N(2)
O(4)–Mo–N(1)
O(4)–Mo–N(2)
N(1)–Mo–N(2)
155.77(12)
93.59(13)
103.51(15)
81.25(11)
84.36(12)
96.96(13)
93.61(13)
82.43(12)
73.88(12)
108.92(14)
163.24(11)
90.24(12)
87.82(12)
158.49(13)
73.44(11)
155.51(10)
94.05(10)
102.62(11)
81.03(9)
103.2(2)
80.93(18)
84.0(2)
84.19(10)
97.38(10)
94.11(10)
82.01(9)
96.6(2)
94.1(2)
83.04(19)
73.98(19)
109.1(2)
163.3(2)
90.6(2)
74.15(9)
108.75(12)
163.82(10)
90.58(10)
87.40(10)
158.74(10)
73.64(9)
87.6(2)
158.3(2)
73.26(19)
Appl. Organometal. Chem. 2016, 30, 116–124
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

S. K. Kurapati and S. Pal
a
Table 3. Hydrogen bonding parameters
attributed to a ligand-centred transition. Similar pairs of absorption
bands observed for complexes of cis-dioxomolybdenum(VI) with
ligands having comparable donor atoms are reported to be due
to ligand-to-metal charge transfer and ligand-centred transitions,
Complex
D–HꢁꢁꢁA
HꢁꢁꢁA (Å)
DꢁꢁꢁA (Å)
D–HꢁꢁꢁA (°)
i
1
N(1)–H(1A)ꢁꢁꢁO(1)
N(2)–H(2A)ꢁꢁꢁO(2)
N(1)–H(1A)ꢁꢁꢁO(2)
N(2)–H(2A)ꢁꢁꢁO(3)
N(1)–H(1A)ꢁꢁꢁO(2)
N(2)–H(2A)ꢁꢁꢁO(3)
N(1)–H(1A)ꢁꢁꢁO(2)
N(2)–H(2A)ꢁꢁꢁO(3)
2.42
2.04
1.98
2.03
1.99
2.02
1.96
2.03
3.316(3)
2.931(3)
2.880(4)
2.917(4)
2.890(3)
2.924(3)
2.866(6)
2.920(6)
167.6
164.3
171.8
163.5
171.2
169.4
176.1
165.1
ii
[38,48–53,58–60]
respectively.
iii
iv
v
1
13
13
2
3
4
H NMR, C NMR and C-DEPT NMR spectra (Figs. S4–S6,
supporting information) of 1–5 in dimethylsulfoxide-d were
6
recorded. All the data with tentative assignments are listed in the
vi
vii
viii
1
Experimental section. The H NMR spectra of 1–5 (Fig. S4) are consis-
tent with the same gross molecular structure for all (Scheme 1; Figs. 1
and S1). A doublet appearing in the range 6.57 ꢀ 6.70 ppm is attrib-
2
3
uted to the proton at C , whereas the proton at C appears as a trip-
a
Symmetry transformations used: (i) x ꢀ 1/2, ꢀy + 1, z; (ii) x + 1/2, ꢀy
let for 1 at 7.12 ppm and as a doublet for 2–5 at 6.58–7.27 ppm. The
+
+
+
1, z; (iii) ꢀ x, ꢀy + 2, ꢀz; (iv) ꢀ x, ꢀy + 1, ꢀz; (v) ꢀ x, ꢀy + 1, ꢀz
1; (vi) ꢀ x, ꢀy, ꢀz + 1; (vii) ꢀ x + 1, ꢀy + 2, ꢀz + 1; (viii) ꢀ x + 1, ꢀy
1, ꢀz + 1.
5
proton attached to C resonates as a doublet for 1 and as a singlet
for 2–5 at 6.69–7.30 ppm. But in the case of 3, the doublet corre-
2
5
sponding to the C proton overlaps with the singlet due to the C
4
proton. The proton at C appears as a triplet at 6.74 ppm for 1.
The protons of the Me and OMe substituents at C in 2 and 3
4
resonate as a singlet at 2.22 and 3.68 ppm, respectively. The
diastereotopicity of the methylene protons is exhibited by the
spectrum of each of 1–5. Two doublets observed at ca 4.56 and
ca 3.77 ppm are ascribed to the protons of the methylene group
7
8
(
C H
plets at ca 3.57 and ca 2.96 ppm. Similarly the C H
and 5 resonate as two multiplets at ca 2.68 and ca 2.60 ppm,
while those of 2 and 3 show a single two-proton multiplet at ca
.60 ppm. A two-proton multiplet observed at 2.41 ppm for all
2 2
) ortho to phenolate. The C H protons appear as two multi-
9
2
protons of 1,
4
2
10
2
the complexes is assigned to the C H protons. Two multiplets
Figure 2. N–HꢁꢁꢁO hydrogen bonding-assisted one-dimensional assemblies
1
2
2 2
of cis-[MoO (L )] (1) (top) and cis-[MoO (L )] (2) (bottom).
Spectroscopic characteristics
The infrared spectra of 1–5 were recorded using KBr pellets. A
few selected bands out of a large number of bands observed in
ꢀ1
the range 400–4000 cm have been assigned. Complexes 1–4
ꢀ
1
display two broad bands at ca 3215 and ca 3150 cm , while 5
ꢀ
1
shows a single broad band at 3205 cm . These bands are attrib-
n 2ꢀ
uted to the two metal-coordinated NH fragments of (L ) . The
aromatic and aliphatic C–H stretches appear as multiple bands
ꢀ
1
in the range 2830–2970 cm . The symmetric and asymmetric
2+
stretches of the {cis-MoO2} core are observed as two strong
ꢀ
1
and sharp bands in the ranges 914–926 and 876–887 cm
respectively.
,
[
38,48–53,58–63]
Electronic absorption spectra of 1–5 were recorded using their
dimethylsulfoxide solutions. The spectral profiles of all the com-
plexes are quite similar (Fig. S3, supporting information). Each com-
plex displays two major absorption bands below 350 nm. The first
band appears in the range 313–340 nm, while the second band is
n
observed at 273–301 nm. In comparison, free H L in methanol
2
display a strong band in the wavelength range 276–297nm. Thus
for 1–5 the first low-energy band is assigned to ligand-to-metal
charge transfer transition, while the second higher energy band is
n
Scheme 2. cis-[MoO
2
(L )] (1–5) catalysed oxidative bromination of various
olefinic compounds employing the optimized reaction conditions described
in the text.
wileyonlinelibrary.com/journal/aoc
Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2016, 30, 116–124

2
+
cis-{MoO2} complexes and their bromoperoxidase activities
1
1
observed at ca 4.50 and ca 4.40 ppm are assigned to the C H
protons. However, H and H
2
corresponding to the carbons of the Me and the OMe substituents
at para position of the phenolate ring of (L ) in 2 and 3 are
7a
11a
n 2ꢀ
provide overlapping signals in
the cases of 4 and 5. The protons at the nitrogen atoms of the
benzylamine and diethyleneamine fragments are observed as
two singlets in the ranges 5.73–5.83 and 5.32–5.50 ppm, respec-
tively. Overall trends in the proton chemical shifts are consistent
with the nature of the para-substituents of the phenolate moiety
observed at 21.05 and 56.23 ppm, respectively. The spectral assign-
ments described above are also consistent with the C-DEPT NMR
spectra of 1–5 (Fig. S6).
13
Redox properties
n 2ꢀ
of (L ) in 1–5.
Cyclic voltammetry was used to scrutinize the redox properties
1
3
The C NMR spectra of 1–5 (Fig. S5) are also in good agreement
with their similar molecular structures (Scheme 1). Generally the
chemical shifts of the individual carbon atoms of the phenolate ring
vary significantly with the change of its para substituent, whereas
there is essentially no change in the chemical shift of each of the
five methylene carbon atoms in the remaining aliphatic skeleton
of 1–5. The measurements were performed under nitrogen atmo-
sphere at 298 K with ca 10 M solutions of the corresponding
complexes in dimethylformamide containing tetrabutylammonium
perchlorate as the supporting electrolyte. The three-electrode setup
comprised a Pt disc working electrode, a Pt wire auxiliary electrode
ꢀ3
and an Ag/AgCl reference electrode. The ferrocenium/ferrocene
n 2ꢀ
of (L ) . Six resonances observed in the ranges 153.07–161.15,
+
(
Fc /Fc) couple was observed at E1 = 0.66 V under identical condi-
/2
1
1
19.28–121.82, 114.57–131.82, 110.73–155.13, 115.56–132.89 and
20.08–126.55 ppm are assigned to the phenolate ring carbons
tions. The cyclic voltammograms are shown in Fig. S7 (supporting
information). All of 1–5 display an irreversible reduction response
with cathodic peak potentials (Epc) in the range ꢀ0.73 to ꢀ0.88 V.
The one-electron nature of these responses is inferred
1
6
C –C , respectively. The aliphatic resonances appearing at ca
3.60, ca 47.59, ca 47.27, ca 52.05 and ca 73.73 ppm are attributed
5
7
11
to the methylene carbon atoms C –C , respectively. The signals
+
by comparing the cathodic peak currents with that of the Fc /Fc
[38,52,64]
couple as well as with other known one-electron processes.
Such irreversible cathodic responses observed for complexes of
cis-dioxomolybdenum(VI) with ligands having similar type of coor-
dinating atoms have been reported to be due to molybdenum(VI)
[35,38,48,49,52,60]
to molybdenum(V) reduction.
Catalytic studies
Bromoperoxidase activities of 1–5 were examined using oxidative
bromination of a variety of olefinic compounds and salicylaldehyde
in presence of H O , KBr and HClO . The product selectivity patterns
2
2
4
are summarized in Scheme 2. Catalytic abilities of 1–5 in benzoin
and methyl(phenyl)sulfide oxidation reactions with H O were also
2
2
n
investigated. The results are shown in Scheme 3. It is very likely
that an oxoperoxomolybdenum(VI) species produced in the reac-
Scheme 3. cis-[MoO
2
(L )] (1–5) catalysed oxidation of benzoin and
methyl(phenyl)sulfide under optimized reaction conditions described in
the text.
n
tion of cis-[MoO (L )] with H O plays a key role in the catalytic
2
2 2
a
Table 4. Optimization of oxidative bromination reaction using 3 as catalyst
ꢀ
1
)
Catalyst (mmol)
2
Solvent–H O (1:1)
H O
2 2
(mmol)
Conversion (%)
Time (h)
TOF (h
Product selectivity (%)
(
a)
(b)
48
79
41
41
48
41
40
41
50
45
40
48
45
41
39
36
70
(c)
14
12
12
10
10
8
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
.01
.01
.01
.01
.005
.01
.02
.05
.01
.01
.01
.01
.01
.01
.01
.01
EtOH
ACN
20
20
20
20
20
20
20
20
20
20
20
5
99
70
1
990
700
34
—
36
38
30
45
36
40
28
35
38
28
30
32
35
38
30
1
CHCl
CH Cl
3
50
1
500
2
2
54
1
540
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
58
1
1160
990
99
1
>99
>99
45
1
445
14
16
16
18
18
18
20
21
15
15
—
1
198
0.25
0.5
1
1800
1980
990
99
>99
40
0.5
0.5
0.5
0.5
0.5
—
800
10
15
20
30
15
60
1200
1960
1980
1980
—
98
>99
>99
10
Blank
a
4
Conditions used: solvent mixture (6 ml), styrene (10 mmol), KBr (20 mmol), HClO (20 mmol), temperature 298 K.
Appl. Organometal. Chem. 2016, 30, 116–124
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

S. K. Kurapati and S. Pal
a
bromoperoxidation as well as benzoin and methyl(phenyl)sulfide
oxidation reactions.
Table 5. Data for catalytic oxidative bromination
[27–31,65]
Substrate
Conversion Time
TOF
Product selectivity
(%)
ꢀ
1
and catalyst
(%)
(h)
(h )
Oxidative bromination
Styrene
(a)
37
35
32
35
36
(d)
26
26
28
35
31
(g)
52
55
56
50
51
(j)
(b)
40
43
41
40
42
(e)
44
48
48
45
46
(h)
32
28
28
30
30
(k)
37
37
36
35
39
(n)
33
30
27
31
32
(q)
33
30
26
35
36
(t)
(c)
18
19
21
19
18
(f)
Screening of solvents, oxidant (H
2
O
2
) and catalyst loading to opti-
1
94
98
98
94
96
0.5
1880
1960
1960
1880
1920
mize the reaction conditions at room temperature was done using
as catalyst and styrene as substrate. Reaction time was decided by
2
0.5
0.5
0.5
0.5
3
3
considering the maximum conversion of styrene. Various solvents
such as methanol, ethanol, acetonitrile, dichloromethane and chlo-
roform with water (in 1:1 ratio) as co-solvent were investigated. The
substrate conversion is low (ca 50%) in each of chloroform–water
and dichloromethane–water mixtures, while it improves consider-
ably (ca 70%) in acetonitrile–water mixture but remains incom-
plete. In contrast, the conversion is essentially complete in both
methanol–water and ethanol–water mixtures (Table 4). Reaction
of styrene, H O , KBr and HClO in 1:1.5:2:2 mole ratio in the pres-
4
5
4-Bromostyrene
1
96
97
98
95
96
0.5
0.5
0.5
0.5
0.5
1920
1940
1960
1960
1920
28
18
16
15
18
(i)
2
3
4
5
trans-Anethole
2
2
4
ence of 0.1 mol% catalyst in methanol–water (1:1) are considered
to be the best reaction conditions (Table 4). Only 10% of styrene
is converted to products in the control experiment without catalyst.
The best reaction conditions thus found except for the reaction
time (determined separately for each substrate considering its
maximum conversion) were used for other olefinic substrates. The
products obtained from various substrates using the optimized
reaction conditions and 1–5 as catalysts are shown in Scheme 2,
and the reaction times, conversions and product selectivities are
summarized in Table 5.
1
89
92
94
88
88
0.75
0.75
0.75
0.75
0.75
1187
1227
1253
1173
1173
15
16
16
16
16
(l)
2
3
4
5
Cyclohexene
1
95
96
98
92
93
0.66
0.66
0.66
0.66
0.66
1440
1455
1484
1393
1410
44
43
42
44
39
(m)
50
52
55
50
51
(p)
40
42
42
40
39
(s)
19
16
15
19
18
(v)
71
80
90
82
83
19
20
22
21
22
(o)
17
18
18
19
17
(r)
2
3
4
In the case of styrene, three products, 1,2-diol, 2-bromo-1-ol and
5
1,2-dibromide, are obtained as major products with very small
1-Octene
amount of epoxide (≤6%). In our previous study of oxidative
bromination of styrene using cis-dioxomolybdenum(VI) complexes
1
82
80
82
85
85
1
1
1
1
1
820
800
820
850
850
2
with tetradentate but tripodal NO
-bromo-1-ol the other two main products were 2-bromo-1-one
3
-donor ligands as catalysts, with
3
2
4
[
38]
and epoxide instead of 1,2-diol and 1,2-dibromide. Interestingly,
the same reaction catalysed by cis-dioxomolybdenum(IV) com-
plexes with tridentate ligands has been reported to produce the
same three products as observed here but with different
5
α-Pinene
1
72
75
78
73
75
2.5
2.5
2.5
2.5
2.5
288
300
312
292
300
25
27
30
23
22
(u)
45
45
49
45
45
(x)
16
14
2
2
[29,31]
selectivities.
The TOF values found in our earlier study are less
3
[
38]
compared to those found for 1–5. Complexes from rigid tripodal
4
NO -donor ligands are expected to be more stable and less
5
3
oxophilic compared to 1–5 containing the relatively more flexible
linear ONNO-donor ligand system (L ) . Consequently, 1–5 are
Limonene
n 2ꢀ
1
84
91
95
84
88
1.25
1.25
1.25
1.25
1.25
672
728
760
672
704
35
38
36
35
37
(w)
13
6
more reactive and hence better oxidative catalysts than the earlier
complexes. It may be noted that still higher TOF values have
been reported for cis-dioxomolybdenum(VI) complexes with some
2
3
4
[29]
tridentate ligands. In this type of species the sixth coordination
site is occupied by a solvent molecule which can easily dissociate
to make way for an extra peroxo moiety. When 4-bromostyrene is
used as substrate, the conversions and product selectivities are very
close to those observed for styrene (Table 5). However, epoxides
instead of 1,2-diols are obtained with 2-bromo-1-ols and 1,2-
dibromides for substrates such as trans-anethole, cyclohexene,
5
Salicylaldehyde
1
2
3
4
5
98
98
99
96
97
0.25
0.25
0.25
0.25
0.25
3920
3920
3960
3840
3880
8
12
5
6
12
1
-octene and α-pinene (Scheme 2). In each case, 2-bromo-1-ol is
a
Reaction conditions: MeOH–H
2
O (1:1) mixture (6 ml), substrate (10 mmol),
(20 mmol), H (15 mmol),
produced more selectively than the dibromo derivative and the
epoxide (Table 5). In the case of limonene, it is important to note
that no dibromide is observed and with more selectively pro-
duced 2-bromo-1-ol both mono- and diepoxides were formed in
significant amounts. Including the above mentioned olefinic
compounds, the catalytic oxidative bromination of salicylaldehyde
was also performed. In this reaction, although the products
obtained are similar, 5-bromosalicylaldehyde is formed with very
catalyst (0.01 mmol), KBr (20 mmol), HClO
temperature 298 K.
4
2 2
O
[
38]
including our recent results.
Overall, catalytic efficiencies of
1–5 in all the above oxidative bromination reactions are compara-
ble (Table 5). Thus the variation of substituents (R) on the ligand
n 2ꢀ
(L ) has no effect on the bromoperoxidase activities of the
[29–31]
high selectivity when compared with previous observations
present series of complexes.
wileyonlinelibrary.com/journal/aoc
Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2016, 30, 116–124

2
+
cis-{MoO2} complexes and their bromoperoxidase activities
Benzoin oxidation
Table 7. Data for catalytic benzoin and methyl(phenyl)sulfide oxidation
Complexes 1–5 were also used as catalysts in benzoin oxidation
reaction in methanol. Optimized reaction conditions were deter-
Catalyst
Conversion Time (h)
(%)
TOF
Product selectivity
(%)
ꢀ
1
(h )
2 2
mined by varying the amounts of 3 as catalyst and H O (30%
w/w aqueous solution) as oxidant (Table 6). It is found that
maximum conversion of benzoin occurs in 1 h when reacted with
a
Benzoin oxidation
BA BZL
DMT
18
1
2
3
4
5
96
1
1
1
1
1
960
970
980
940
950
46
48
53
46
49
33
31
28
31
30
97
98
94
95
19
2 2
H O in 1:2 mole ratio in the presence of 0.1 mol% 3. These same
18
reaction conditions were used for each of the remaining four com-
plexes as catalyst. In all cases, benzoic acid is obtained as the major
product with benzil and α,α-dimethoxytoluene as the minor products
21
18
b
Methyl(phenyl)sulfide oxidation
Sulfoxide Sulfone
(Scheme 3; Table 7). It is noted that methyl benzoate is also produced
1
2
3
4
5
92
93
94
91
93
1
1
1
1
1
920
930
940
910
930
67
65
64
66
65
33
35
36
33
35
but in a very small amount (<3%). The present product selectivity
and yield patterns are very similar to those observed in our previous
[38]
study. However, methyl benzoate was reported to be produced in
a much larger amount with the other three products in benzoin oxi-
dation reactions catalysed by cis-dioxomolybdenum(IV) complexes
[29,30]
with tridentate ligands.
In the control experiment without any
a
Reaction conditions: MeOH (6 ml), benzoin (10 mmol), catalyst
(0.01 mmol), H O (20 mmol), temperature 298 K.
catalyst, only 8% of benzoin is converted to benzoic acid (major prod-
uct), benzil (distant second major product) and α,α-dimethoxytoluene
2
2
b
Reaction conditions: methyl(phenyl)sulfide (10 mmol), catalyst
(0.01 mmol), H O (20 mmol), temperature 298 K.
(
very small amount) (Table 7). Regardless of the nature of the substit-
2
2
n 2ꢀ
uent (R) on (L ) , the catalytic performances of 1–5 in the benzoin
oxidation reaction remain comparable.
the remaining four complexes in methyl(phenyl)sulfide oxidation
were assessed using the above mentioned combination of
substrate, oxidant and catalyst. In all cases, two oxidized products,
namely methyl(phenyl)sulfoxide and methyl(phenyl)sulfone, are
formed in a ratio of ca 1.9:1 (Scheme 3; Table 7). Under the opti-
mized reaction conditions without catalyst, 20% of the substrate is
converted to oxidized products. There are few reports of sulfide ox-
idation using tridentate ligand-supported cis-dioxomolybdenum(VI)
Methyl(phenyl)sulfide oxidation
The series of complexes 1–5 was also tested for catalytic activity in
the oxidation of methyl(phenyl)sulfide with H O . The reaction
conditions were optimized by varying the amounts of 3 as catalyst
2
2
and the oxidant H O (30% w/w aqueous solution) (Table 6). A
2
2
combination of methyl(phenyl)sulfide and H O in 1:1.5 mole ratio
in the presence of 0.1mol% 3 is found to be the best for maximum
conversion of substrate to products in 1 h. The catalytic activities of
2
2
[29,60,61]
complexes as catalysts.
When compared with the present
results, the catalytic activities and the product selectivities are
[
29]
similar in one case,
while essentially 100% selectivity for the
[60,61]
sulfoxide was reported in the other cases.
As observed in the
Table 6. Optimization of benzoin and methyl(phenyl)sulfide oxidation
reactions
oxidative bromination and benzoin oxidation, here also the catalytic
activities of 1–5 are practically invariant with the change of substit-
uents on their corresponding ligands.
Catalyst
H
2
O
2
Conversion Time TOF Product selectivity
ꢀ
1
3
(mmol) (mmol)
(%)
(h)
(h
)
(%)
a
Benzoin oxidation
BA
45
52
58
59
49
51
53
58
72
BZL
34
28
24
23
33
30
28
25
24
DMT
18
18
17
19
14
17
18
14
4
Conclusions
0.005
0.01
0.02
0.05
0.01
0.01
0.01
0.01
20
20
20
20
10
15
20
30
20
48
1
480
970
990
990
510
670
980
990
—
97
99
1
1
1
1
1
1
1
1
A new series of complexes having the general molecular formula
n
cis-[MoO
2
(L )] (1–5) with unsymmetric linear tetradentate 2-((2-(2-
n
>99
51
2
hydroxyethylamino)ethylamino)methyl)-4-R-phenols (H L ) have
been synthesized. Molecular structures of four of the complexes
have been determined using X-ray crystallography. In each structure,
67
n 2ꢀ
2+
98
(
L ) coordinates to the cis-{MoO
two secondary amine-N and ethanolate-O atoms and completes
a distorted octahedral N coordination geometry of the metal
2
}
unit through phenolate-O,
>99
8
blank
2 4
O
b
Methyl(phenyl)sulfide oxidation
Sulfoxide Sulfone
centre. Spectroscopic and redox characteristics are consistent with
the same overall molecular structure for all five complexes. All the
complexes are catalytically active in oxidative bromination of
various olefinic compounds and salicylaldehyde and in benzoin
and sulfide oxidation reactions. The product selectivities in each
reaction are very similar for all five complexes. Thus the substitu-
0
0
0
0
0
0
0
0
.005
.01
.02
.05
.01
.01
.01
.01
20
20
20
20
5
52
96
1
1
1
1
1
1
1
1
1
520
960
980
990
540
750
940
970
—
65
60
67
65
64
66
65
65
72
35
40
33
35
36
33
35
35
28
98
>99
54
n 2ꢀ
10
15
20
15
75
ents at the para position of the phenolate fragment of (L ) have
no significant effect on the catalytic abilities of 1–5.
94
97
blank
20
Acknowledgments
a
Conditions used: MeOH (6 ml), benzoin (10 mmol), temperature 298 K.
b
S. K. Kurapati thanks the Council of Scientific and Industrial Re-
search (CSIR), New Delhi for a research fellowship. We also thank
Conditions used: methyl(phenyl)sulfide (10 mmol), temperature 298 K.
Appl. Organometal. Chem. 2016, 30, 116–124
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

S. K. Kurapati and S. Pal
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