Dioxidomolybdenum(VI) complexes bearing sterically constrained aroylazine ligands: Synthesis, structural investigation and catalytic evaluation

Article abstract of DOI:10.1016/j.ica.2017.09.043

Seven new dioxidomolybdenum(VI) complexes [MoO₂L¹(X)].X (1) and [MoO₂L^2–7(X)] (2–7) [Where X = EtOH in case of 1 and 5 and X = DMSO in case of 2–4 and 6, 7] of aroylazines containing a bulky 3-hydroxy-2-naphthoic substituent, were isolated and structurally characterized. The aroylazine ligands H₂L^1–7 were derived from the condensation of 3-hydroxy-2-naphthoic acid hydrazide with several substituted aromatic aldehydes/ketones. All the synthesized ligands and metal complexes were successfully characterized by elemental analysis, IR, UV–Vis and NMR spectroscopy. X-ray structures of 1–6 revealed that the ligands coordinate to the metal center as a dibasic tridentate ligand. Cyclic voltammetry of the complexes shows two irreversible reductive responses within the potential window ?0.50 to ?1.36 V, due to Mo^VI/Mo^V and Mo^V/Mo^IV processes. The synthesized complexes 1–7 were used as catalysts for the oxidation of benzoin, and for the oxidative bromination of salicylaldehyde, as a functional mimic of haloperoxidase. It was found that the percentage of conversion increased significantly in the presence of catalysts 1–7 which contained bulky substituents, and showed high percentage of conversion (>90%) with high turnover frequency (>1100 h^?1) than previously reported catalysts. Benzil, benzoic acid and benzaldehyde-dimethylacetal were formed selectively for the oxidation of benzoin. Formation of 5-bromosalicylaldehyde and 3,5-dibromosalicylaldehyde took place during the oxidative bromination of salicylaldehyde in presence of H₂O₂ as an oxidant and therefore 1–7 act as functional models of vanadium dependent haloperoxidases.

Full text of DOI:10.1016/j.ica.2017.09.043

Accepted Manuscript
Dioxidomolybdenum(VI) complexes bearing sterically constrained aroylazine
ligands: Synthesis, structural investigation and catalytic evaluation
Sudarshana Majumder, Sagarika Pasayat, Satabdi Roy, Subhashree P. Dash,
Sarita Dhaka, Mannar R. Maurya, Martin Reichelt, Hans Reuter, Krzysztof
Brzezinski, Rupam Dinda
PII:
S0020-1693(17)31047-2
http://dx.doi.org/10.1016/j.ica.2017.09.043
ICA 17902
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
5 July 2017
15 September 2017
15 September 2017
Please cite this article as: S. Majumder, S. Pasayat, S. Roy, S.P. Dash, S. Dhaka, M.R. Maurya, M. Reichelt, H.
Reuter, K. Brzezinski, R. Dinda, Dioxidomolybdenum(VI) complexes bearing sterically constrained aroylazine
ligands: Synthesis, structural investigation and catalytic evaluation, Inorganica Chimica Acta (2017), doi: http://
dx.doi.org/10.1016/j.ica.2017.09.043
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Dioxidomolybdenum(VI) complexes bearing sterically constrained aroylazine ligands:
Synthesis, structural investigation and catalytic evaluation
Sudarshana Majumdera, Sagarika Pasayata, Satabdi Roya, Subhashree P. Dasha,b, Sarita
Dhakac,d, Mannar R. Mauryad, Martin Reichelte, Hans Reutere, Krzysztof Brzezinskif, Rupam
Dinda*a
aDepartment of Chemistry, National Institute of Technology, Rourkela 769008, Odisha, India
bDepartment of Basic Sciences, Parala Maharaja Engineering College, Sitalapalli,
Brahmapur, Odisha 761003, India
cCentral Scientific Instruments Organisation (CSIR-CSIO), Sector 30 C, Chandigarh 160030,
India
dDepartment of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India
eInstitute of Chemistry of New Materials, University o , Barbarastrasse 7,
, Germany
f Institute of Chemistry, University of Bialystok, Ciolkowskiego1K,1 15-245 Bialystok, Poland
1

Abstract
Seven new dioxidomolybdenum(VI) complexes [MoO2L1(X)].X (1) and [MoO2L27(X)] (2–
7) [Where X= EtOH in case of 1 and 5 and X=DMSO in case of 24 and 6, 7] of aroylazines
containing a bulky 3-hydroxy-2-naphthoic substituent, were isolated and structurally
characterized. The aroylazine ligands H2L1-7 were derived from the condensation of 3-
hydroxy-2-naphthoic acid hydrazide with several substituted aromatic aldehydes/ketones. All
the synthesized ligands and metal complexes were successfully characterized by elemental
analysis, IR, UV–Vis and NMR spectroscopy. X-ray structures of 16 revealed that the
ligands coordinate to the metal center as a dibasic tridentate ligand. Cyclic voltammetry of
the complexes shows two irreversible reductive responses within the potential window 0.50
to 1.36 V, due to MoVI/MoV and MoV/MoIV processes. The synthesized complexes 1–7 were
used as catalysts for the oxidation of benzoin, and for the oxidative bromination of
salicylaldehyde, as a functional mimic of haloperoxidase. It was found that the percentage of
conversion increased significantly in the presence of catalysts 1–7 which contained bulky
substituents, and showed high percentage of conversion (>90%) with high turnover frequency
(>1100 h-1) than previously reported catalysts. Benzil, benzoic acid and benzaldehyde-
dimethylacetal were formed selectively for the oxidation of benzoin. Formation of 5-
bromosalicylaldehyde and 3,5-dibromosalicylaldehyde took place during the oxidative
bromination of salicylaldehyde in presence of H2O2 as an oxidant and therefore 1–7 act as
functional models of vanadium dependent haloperoxidases.
Keywords: Aroylazines / dioxidomolybdenum(VI) / X-ray crystallography / oxidation of
benzoin / oxidative bromination of salicylaldehyde
2

1. Introduction
The chemistry of dioxidomolybdenum(VI) complexes has continued to attract the attention of
researchers owing to its versatile reactivity [1,2] and biochemical [37] and catalytic [812]
significance. The presence of molybdenum-oxido bonds in various molybdoenzymes [1315,
16, 17] has stimulated research on molybdenum complexes with oxygen environments [2,
1821], and in this aspect molybdenum Schiff-base complexes have gained substantial
importance in recent times [18, 2226]. In particular, aroylazines containing azomethine
group (C=NN=C) [27−29], constitute an interesting category of such ligands as they have
the potential to stabilize metal ions in different oxidation states [2731] and can therefore
tune the molecular architecture and geometry [32] of complexes. Further, the donor/acceptor
abilities, electronic properties and steric strain factors of ligands influence the catalytic
potential of the complexes [32‒37] as evidenced in various reactions catalysed by
molybdenum complexes [8, 3641]. Such reactions catalysed by molybdenum complexes
include the isomerization of allyl alcohols [42], olefin metathesis [43], oxo transfer reactions
[44‒46], ammoxidation of propylene [47] and epoxidation of alkenes [48].
On another hand, oxidation of -hydroxy ketones to -diketones belong to one of the most
significant classes of organic transformations [49,50]. The dicarbonyl compounds like
benzils, obtained from oxidation of benzoin, can further be employed in the synthesis of
biologically active organic molecules [5154] and as photosensitive agents. Although several
oxidizing agents like chromium oxide [55,56], nitric acid [57], ammonium nitrate–copper
acetate [58], thallium nitrate [59], triarylstibanes [60], etc. have been explored for catalysing
these reactions, the search of versatile reagents is still prevalent for the development of
convenient and selective procedures. However, very few reports of oxidomolybdenum
complexes catalysed benzoin oxidation reactions are available in the literature [32, 6063].
3

Additionally, haloperoxidase reactions are environmentally benign procedures for
brominating organic compounds [6467]. Haloperoxidase enzymes catalyse the oxidation of
halide ions by hydrogen peroxide [6668] and yield halogenated organic substrates which
have pharmacological significance due to their antifungal, antibacterial, antineoplastic,
antiviral and anti-inflammatory properties [67,6971]. There has been extensive studies on
oxidovanadium(V) complexes which can act as haloperoxidase mimics and several vanadium
dependant bromoperoxidase models have been established [7281]. However, reports of
oxidomolybdenum(VI) haloperoxidase models are scanty [32,63,82,83] and seek further
exploration.
As a part of our continuing interest in oxidometal complexes of aroylazines in relation to
their biological [8492] and catalytic [32,84,93] applications, herein we report seven
dioxidomolybdenum(VI) complexes [MoO2L1(X)].X (1) and [MoO2L27(X)] (2–7) [Where
X= EtOH in case of 1 and 5 and X=DMSO in case of 24 and 6, 7] of aroylazine ligands
H2L1-7 (Scheme 1). Earlier we had explored the catalytic potential of
dioxidomolybdenum(VI) complexes of similar aroylazines where fair catalytic results were
obtained [32]. To improve the catalytic efficiency, a bulky 3-hydroxy-2-naphthoic substituent
has now been introduced in aroylazine ligands to observe its influence, if any, on the catalytic
potential of the reported complexes. All the complexes have been characterized by several
physicochemical techniques (IR, UV–Vis, NMR and ESI-MS) and the crystal structures of
[MoO2L1(EtOH)].EtOH
(1),
[MoO2L2(DMSO)]
(2),
[MoO2L3(DMSO)]
(3),
[MoO2L4(DMSO)] (4), [MoO2L5(EtOH)] (5) and [MoO2L6(DMSO)] (6) have been
determined by X-ray crystallography. The catalytic activity of 1–7 for the oxidation of
benzoin, and for the oxidative bromination of salicylaldehyde, as a functional mimic of
haloperoxidase has been explored. Significant percentage of conversion (>90%) was obtained
4

in the presence of catalysts 1–7, and better selectivity of products and high turnover
frequency (>1100 h-1) was achieved in comparison to previously reported catalysts
[32,60,63,82,83]. During the oxidation of benzoin, benzil, benzoic acid and benzaldehyde-
dimethylacetal were formed selectively. Also, selective formation of 5-bromosalicylaldehyde
and 3,5-dibromosalicylaldehyde took place during the oxidative bromination of
salicylaldehyde in presence of H2O2 as an oxidant and therefore 1–7 acted as functional
models of vanadium dependent haloperoxidases.
5

2. Experimental Section
2.1. Materials and methods
[MoO2(acac)2] was prepared as described in the literature [94]. Reagent grade solvents were
dried and distilled prior to use. All other chemicals were reagent grade, available
commercially and used as received. Elemental analyses were performed on a VarioELcube
CHNS Elemental analyzer. IR spectra were recorded on a Perkin-Elmer Spectrum RXI
spectrometer. NMR spectra were recorded with a Bruker Ultrashield 400 MHz spectrometer
13
at 298 K room temperature using SiMe4 (1H and C) as an internal. Electronic spectra were
recorded on a Lamda25, PerkinElmer spectrophotometer. Mass spectra were recorded on an
SQ-300 MS instrument operating in ESI mode. Magnetic susceptibility was measured with a
Sherwood Scientific AUTOMSB sample magnetometer. Conductivity was measured using
Eutech CON 700 conductivity meter. A CH-Instruments (Model No. CHI6003E)
electrochemical analyzer was used for cyclic voltammetric experiments with CH3CN
solutions of the complexes containing TBAP (tetra butyl ammonium perchlorate) as the
supporting electrolyte. The three electrode measurements were carried out at 298 K with a
glassy carbon working electrode, platinum auxiliary electrode, and Ag/AgCl as a reference
electrode. CAUTION: Although no problems were encountered during the course of this
work, attention is drawn to the potentially explosive nature of perchlorates.
A Shimadzu 2010 plus gas-chromatograph fitted with a Rtx-1 capillary column (30 m × 0.25
mm × 0.25 μm) and a FID detector was used to analyse 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:
6

Peak area of a substrate
% Conversion of substrate
X 100
=
100
Total area of substrate + products
Peak area of a product
X 100
% Selectivity of a product =
Total area of products
2.2. Synthesis of Ligands (H2L1–7)
The aroylazine ligands, H2L1–7, were prepared by the condensation of 3-hydroxy-2-naphthoic
hydrazide with the corresponding aldehydes and ketones (salicylaldehyde, o-vanillin, 5-
bromosalicylaldehye, 2-hydroxy-1-naphthaldehyde, 2'-hydroxyacetophenone, 2'-hydroxy-4'-
methoxyacetophenone and 2-hydroxy-5-methoxybenzaldehyde, respectively) following a
standard procedure [32]. The resulting light brown compounds were filtered, washed with
ethanol and dried over fused CaCl2.
H2L1: Yield: 0.21 g (68%). Anal. Calcd for C18H14N2O3: C, 70.58; H, 4.61; N, 9.15. Found:
C, 70.54; H, 4.59; N, 9.10 %. FTIR (KBr, νmax/cm–1): ν(O–H) 3540, 3488, ν(N–H) 3003,
ν(C=O) 1645, ν(C=N) 1614. 1H NMR (400 MHz, DMSO-d6): δ (ppm) = 12.15 (s, 1H, NH),
13
11.22 (s, 2H, OH), 8.69 (s, 1H, CH), 8.476.92 (m, 10H, aromatic). C NMR (100 MHz,
DMSO-d6): δ (ppm) = 164.09, 157.99, 154.53, 149.36, 136.39, 132.09, 130.79, 129.95,
129.14, 128.78, 127.24, 126.33, 124.31, 120.39, 119.88, 119.12, 116.93, 111.08.
H2L2: Yield: 0.22 g (64%). Anal. Calcd for C19H16N2O4: C, 67.85; H, 4.79; N, 8.33. Found:
C, 67.81; H, 4.72; N, 8.30 %. FTIR (KBr, νmax/cm–1): ν(O–H) 3554, 3465, ν(N–H), 3005,
ν(C=O) 1648, ν(C=N) 1610. 1H NMR (400 MHz, DMSO-d6): δ (ppm) = 11.94 (s, 1H, NH),
10.92 (s, 2H, OH), 8.69 (s, 1H, CH), 8.456.86 (m, 9H, aromatic), 3.82 (s, 3H, OCH3).
13C NMR (100 MHz, DMSO-d6): δ (ppm) = 164.05, 154.45, 149.12, 148.41, 147.64, 136.34,
7

130.79, 129.14, 128.79, 126.31, 124.32, 121.16, 120.53, 119.61, 119.34, 114.39, 114.02,
56.28.
H2L3: Yield: 0.27 g (70%). Anal. Calcd for C18H13BrN2O3: C, 56.12; H, 3.40; N, 7.27. Found:
C, 56.15; H, 3.42; N, 7.23 %. FTIR (KBr, νmax/cm–1): ν(O–H) 3543, 3398, ν(N–H) 3008,
ν(C=O) 1649, ν(C=N) 1612. 1H NMR (400 MHz, DMSO-d6): δ (ppm) = 11.99 (s, 1H, NH),
13
11.28 (s, 2H, OH), 8.21 (s, 1H, CH), 7.926.92 (m, 9H, aromatic). C NMR (100 MHz,
DMSO-d6): δ (ppm) = 164.22, 156.92, 154.39, 146.62, 136.34, 134.27, 130.84, 129.14,
128.81, 127.21, 126.31, 124.32, 121.71, 120.61, 119.18, 111.00, 110.97.
H2L4: Yield: 0.22 g (62%). Anal. Calcd for C22H16N2O3: C, 74.15; H, 4.53; N, 7.86. Found:
C, 74.16; H, 4.51; N, 7.82 %. FTIR (KBr, νmax/cm–1): ν(O–H) 3548, 3421, ν(N–H) 3007,
ν(C=O) 1645, ν(C=N) 1614. 1H NMR (400 MHz, DMSO-d6): δ (ppm) = 12.80 (s, 1H, NH),
12.25 (s, 1H, OH), 11.36 (s, 1H, OH), 9.57 (s, 1H, CH), 8.537.28 (m, 12H, aromatic).
13C NMR (100 MHz, DMSO-d6): δ (ppm) = 163.49, 158.62, 154.32, 148.03, 136.41, 133.41,
132.16, 131.18, 129.42, 129.20, 128.84, 128.29, 128.24, 127.30, 126.35, 124.37, 124.06,
121.50, 120.44, 119.38, 111.07, 109.10.
H2L5: Yield: 0.19 g (61%). Anal. Calcd for C19H16N2O3: C, 71.24; H, 5.03; N, 8.74. Found:
C, 71.21; H, 5.05; N, 8.71 %. FTIR (KBr, νmax/cm–1): ν(O–H) 3533, 3478, ν(N–H) 3005,
1
ν(C=O) 1642, ν(C=N) 1614. H NMR (400 MHz, DMSO-d6): 13.24 (s, 1H, NH), 11.75 (s,
2H, OH), 8.62  6.91 (m, 10H, aromatic), 3.36 (s, 3H, CH3). 13C NMR (100 MHz, DMSO-
d6): δ (ppm) = 162.16, 159.00, 156.37, 153.10, 136.24, 132.58, 131.64, 129.29, 128.86,
128.76, 127.49, 126.13, 124.30, 120.58, 119.60, 118.99, 117.68, 111.01, 13.60.
H2L6: Yield: 0.23 g (65%). Anal. Calcd for C20H18N2O4: C, 68.56; H, 5.18; N, 8.00. Found:
C, 68.54; H, 5.17; N, 8.05 %. FTIR (KBr, νmax/cm–1): ν(O–H) 3493, 3387, ν(N–H) 3003,
ν(C=O) 1644, ν(C=N) 1615.1H NMR (400 MHz, DMSO-d6): δ (ppm) = 13.55 (s, 1H, NH),
11.69 (s, 2H, OH), 8.61 6.49 (m, 9H, aromatic), 3.79 (s, 3H, OCH3), 2.43 (s, 3H, CH3).
8

13C NMR (100 MHz, DMSO-d6): δ (ppm) = 162.27, 162.04, 161.14, 156.78, 153.22, 136.28,
132.54, 130.25, 129.38, 128.83, 127.58, 126.24, 124.40, 120.74, 112.96, 111.06, 106.27,
102.06, 55.71, 13.74.
H2L7: Yield: 0.24 g (72%). Anal. Calcd for C19H16N2O4: C, 67.85; H, 4.79; N, 8.33. Found:
C, 67.83; H, 4.75; N, 8.32 %. FTIR (KBr, νmax/cm–1): ν(O–H) 3556, 3433, ν(N–H), 3004,
ν(C=O) 1646, ν(C=N) 1614. 1H NMR (400 MHz, DMSO-d6): δ (ppm) = 12.16 (s, 1H, NH),
11.34 (s, 1H, OH), 10.65 (s, 1H, OH), 8.66 (s, 1H, CH), 8.466.88 (m, 9H, aromatic),
13
3.74 (s, 3H, OCH3). C NMR (100 MHz, DMSO-d6): δ (ppm) = 164.07, 154.52, 152.58,
151.99, 148.36, 136.32, 130.73, 129.12, 128.73, 127.20, 126.33, 124.28, 120.64, 119.39,
118.99, 117.79, 112.33, 110.99, 55.93.
2.3. Synthesis of Complexes 17
[MoO2L1(EtOH)].EtOH (1): To the refluxing solution of ligand H2L1 (1 mmol) in 30 mL
ethanol, MoO2(acac)2 (1 mmol) was added. The color of the solution immediately turned red.
After 3 h reflux, the solution was cooled, filtered and kept for crystallization. Slow
evaporation of the filtrate for 2 days produced dark yellow crystals suitable for X-Ray
analysis. Yield: 0.27 g (57 %). Anal. Calcd for C22H24MoN2O7: C, 50.39; H, 4.61; N, 5.34.
Found: C, 50.37; H, 4.60; N, 5.32 %. FTIR (KBr, νmax/cm–1): ν(O–H) 3361, ν(C=N) 1601,
ν(M=O) 918, 910. UV-Vis (DMSO): λmax, nm (ε, dm3 mol-1 cm-1): 417 (1447), 331 (5540),
1
259 (6113). H NMR (400 MHz, DMSO-d6): δ (ppm) = 11.24 (s, 1H, OH), 9.18 (s, 1H,
CH), 8.547.00 (m, 10H, aromatic), 4.43(1H, OH, ethanol), 2.50 (q, 2H, CH2 ethanolic), 1.05
13
(t, 3H, CH3 ethanolic). C NMR (100 MHz, DMSO-d6): δ (ppm) = 208.57, 169.16, 159.74,
157.37, 154.58, 136.92, 136.00, 134.99, 131.52, 129.49, 129.23, 127.43, 126.39, 124.32,
122.54, 120.45, 119.14, 116.13, 111.36, 89.09. ESI-MS: m/z 524.20 [M]+.
9

[MoO2L2(DMSO)] (2): To the refluxing solution of ligand H2L2 (1 mmol) in 30 mL ethanol,
MoO2(acac)2 (1 mmol) was added. The color of the solution immediately turned red. After 3
h of reflux, the solution was cooled, filtered. The dark brown residue obtained was
crystallized in DMSO. Slow evaporation of the solution for 2 days produced orange crystals
suitable for X-Ray analysis. Yield: 0.31 g (58 %). Anal. Calcd for C21H20MoN2O7S: C, 46.67;
H, 3.73; N, 5.18; S, 5.93. Found: C, 46.65; H, 3.77; N, 5.16; S, 5.90 %. FTIR (KBr, νmax/cm–
1): ν(O–H) 3440, ν(C=N) 1615, ν(M=O) 940, 914. UV-Vis (DMSO): λmax, nm (ε, dm3 mol-1
cm-1): 382 (1760), 333 (2853), 260 (3340). 1H NMR (400 MHz, DMSO-d6): δ (ppm) = 11.24
(s, 1H, OH), 9.18 (s, 1H, CH), 8.557.07 (m, 9H, aromatic), 3.83 (s, 3H, OCH3), 2.50 (s,
13
6H, DMSO). C NMR (100 MHz, DMSO-d6): δ (ppm) = 169.18, 157.38, 154.60, 149.52,
148.90, 136.92, 131.52, 129.49, 129.23, 127.42, 126.40, 125.88, 124.31, 122.45, 120.71,
117.98, 116.14, 111.36, 56.36, 40.74. ESI-MS: m/z 540.21 [M]+.
[MoO2L3(DMSO)] (3): To the refluxing solution of ligand H2L3 (1 mmol) in 30 mL ethanol,
MoO2(acac)2 (1 mmol) was added. The color of the solution immediately turned red. After 3
h of reflux, the solution was cooled, filtered. The dark brown residue obtained was
crystallized in DMSO. Slow evaporation of the solution for 2 days produced orange crystals
suitable for X-Ray analysis. Yield: 0.32 g (55 %). Anal. Calcd for C20H17BrMoN2O6S: C,
40.76; H, 2.91; N, 4.75; S, 5.44. Found: C, 40.72; H, 2.89; N, 4.70; S, 5.41. %. FTIR (KBr,
νmax/cm–1: ν(O–H) 3491, ν(C=N) 1608, ν(M=O) 933, 914. UV-Vis (DMSO): λmax, nm (ε, dm3
mol-1 cm-1): 401 (3053), 336 (9933), 261 (11760). 1H NMR (400 MHz, DMSO-d6): δ (ppm) =
11.16 (s, 1H, OH), 9.12 (s, 1H, CH), 8.546.98 (m, 9H, aromatic), 2.50 (s, 6H, DMSO).
13C NMR (100 MHz, DMSO-d6): δ (ppm) = 169.65, 158.93, 156.19, 154.54, 137.99, 137.01,
136.45, 131.72, 129.53, 129.34, 127.44, 126.41, 124.37, 122.40, 121.51, 115.98, 113.16,
111.42, 40.76. ESI-MS: m/z 589.50 [M]+.
10

[MoO2L4(DMSO)] (4): To the refluxing solution of ligand H2L4 (1 mmol) in 30 mL ethanol,
MoO2(acac)2 (1 mmol) was added. The color of the solution immediately turned red. After 3
h of reflux, the solution was cooled, filtered. The dark brown residue obtained was
crystallized in DMSO. Slow evaporation of the solution for 2 days produced orange crystals
suitable for X-Ray analysis. Yield: 0.29 g (52 %). Anal. Calcd for C24H20MoN2O6S: C, 51.43;
H, 3.60; N, 5.00; S, 5.72. Found: C, 51.40; H, 3.62; N, 5.04; S, 5.70 %. FTIR (KBr, νmax/cm–
1: ν(O–H) 3440, ν(C=N) 1587, ν(M=O) 937, 917. UV-Vis (DMSO): λmax, nm (ε, dm3 mol-1
1
cm-1): 428 (1193), 403 (980), 341 (2300), 260 (4580). H NMR (400 MHz, DMSO-d6): δ
(ppm) = 11.29 (s, 1H, OH), 10.15 (s, 1H, CH), 8.647.24 (m, 12H, aromatic), 2.50 (s, 6H,
DMSO). 13C NMR (100 MHz, DMSO-d6): δ (ppm) = 168.23, 160.87, 154.70, 153.95,
151.23, 147.35, 136.89, 133.05, 131.37, 121.49, 129.38, 129.13, 129.05, 127.44, 126.38,
125.45, 124.22, 122.37, 120.75, 116.14, 112.14, 111.46, 40.83. ESI-MS: m/z 560.15 [M]+.
[MoO2L5(EtOH)] (5): To the refluxing solution of ligand H2L5 (1 mmol) in 30 mL ethanol,
MoO2(acac)2 (1 mmol) was added. The color of the solution immediately turned red. After 3
h of reflux, the solution was cooled, filtered. Dark yellow crystals were obtained on slow
evaporation of the filtrate after two days. Slow evaporation of the solution for 2 days
produced orange crystals suitable for X-Ray analysis. Yield: 0.26 g (53 %). Anal. Calcd for
C21H20MoN2O6: C, 51.23; H, 4.09; N, 5.69. Found: C, 51.21; H, 3.60; N, 5.65 %. FTIR (KBr,
νmax/cm–1: ν(O–H) 3310, ν(C=N) 1594, ν(M=O) 917, 902. UV-Vis (DMSO): λmax, nm (ε, dm3
1
mol-1 cm-1): 395 (6293), 327 (17846), 268 (18160). H NMR (400 MHz, DMSO-d6): δ (ppm)
= 11.44 (s, 1H, OH), 8.577.00 (m, 10H, aromatic), 4.35 (1H, OH, ethanol ), 3 .45 (q, 2H,
13
CH2 ethanolic), 3.43 (s, 3H, CH3), 1.06 (t, 3H, CH3 ethanolic). C NMR (100 MHz, DMSO-
d6): δ (ppm) = 163.55, 157.65, 136.62, 132.84, 132.17, 129.78, 129.66, 129.59, 129.39,
11

129.08, 128.55, 127.17, 126.99, 126.30, 125.60, 124.34, 124.17, 117.02, 103.93, 12.10. ESI-
MS: m/z 492.02 [M]+.
[MoO2L6(DMSO)] (6): To the refluxing solution of ligand H2L6 (1 mmol) in 30 mL ethanol,
MoO2(acac)2 (1 mmol) was added. The color of the solution immediately turned red. After 3
h of reflux, the solution was cooled, filtered. The dark brown residue obtained was
crystallized in DMSO. Slow evaporation of the solution for 2 days produced orange crystals
suitable for X-Ray analysis. Yield: 0.27 g (48 %). Anal. Calcd for C22H22MoN2O7S: C, 47.66;
H, 4.00; N, 5.05; S, 5.78. Found: C, 47.61; H, 4.05; N, 5.00; S, 5.76 %. FTIR (KBr, νmax/cm–
1: ν(O–H) 3454, ν(C=N) 1614, ν(M=O) 926, 904. UV-Vis (DMSO): λmax, nm (ε, dm3 mol-1
cm-1): 400 (1106), 332 (2346), 259 (3320). 1H NMR (400 MHz, DMSO-d6): δ (ppm) = 11.47
(s, 1H, OH), 8.526.58 (m, 9H, aromatic), 3.84 (s, 3H, OCH3), 2.77 (s, 3H, CH3), 2.50 (s,
13
6H, DMSO). C NMR (100 MHz, DMSO-d6): δ (ppm) = 167.19, 164.77, 164.15, 157.56,
154.71, 136.78, 133.17, 131.22, 129.46, 129.12, 127.44, 126.39, 124.28, 116.46, 115.94,
111.13, 110.17, 103.40, 56.29, 47.39, 17.64. ESI-MS: m/z 554.02 [M]+.
[MoO2L7(DMSO)] (7): To the refluxing solution of ligand H2L7 (1 mmol) in 30 mL ethanol,
MoO2(acac)2 (1 mmol) was added. The color of the solution immediately turned red. After 3
h of reflux, the solution was cooled, filtered. The dark brown residue obtained was
crystallized in DMSO. Red microcrystalline residue was obtained. Yield: 0.28 g (52 %).
Anal. Calcd for C21H20MoN2O7S: C, 46.67; H, 3.73; N, 5.18; S, 5.93. Found: C, 46.61; H,
3.70; N, 5.14; S, 5.90 %. FTIR (KBr, νmax/cm–1: ν(O–H) 3491, ν(C=N) 1615, ν(M=O) 966,
942. UV-Vis (DMSO): λmax, nm (ε, dm3 mol-1 cm-1): 402 (3177), 334 (10326), 261 (12206).
1H NMR (400 MHz, DMSO-d6): δ (ppm) = 11.26 (s, 1H, OH), 9.10 (s, 1H, CH),
13
8.535.74 (m, 9H, aromatic), 3.79 (s, 3H, OCH3), 2.50 (s, 6H, DMSO). C NMR (100
12

MHz, DMSO-d6): δ (ppm) = 169.38, 157.02, 154.61, 154.19, 136.93, 131.54, 129.49, 129.22,
127.43, 126.39, 124.31, 122.61, 120.56, 119.92, 117.38, 116.20, 111.55, 111.33, 56.19,
40.78. ESI-MS: m/z 540.85 [M]+.
2.4. Crystallography
2.4.1. General handling and description of equipment
Appropriate single-crystals were selected under a polarization microscope. Inherent
impurities of other crystals were removed carefully and appropriate size was achieved by
cutting. Therefore, crystal shape and size given in the table of crystallographic data only
approximate the properties of the crystal measured but do not reflect habit and size of the
naturally grown crystals. Crystals of 1 was mounted on a 100 m round LithoLoop
(Molecular Dimensions) using Paratone N (Hampton Research) while those of 26 were
mounted on a 50 μm MicroMesh MiTeGen MicromountTM using FROMBLIN Y
perfluoropolyether (LVAC 16/6, Aldrich) before they were centred on a Bruker Kappa APEX
II CCD-based 4-circle X-ray diffractometer using graphite monochromated Mo Kα radiation
(λ = 0.71073 Å) of a fine focus molybdenum-target X-ray tube operating at 50 kV and 30
mA. Crystal cooling was achieved with Cobra, the non-liquid nitrogen Cryostream device
(Oxford Cryosystems) in case of 1 and a Kryoflex low temperature device in case of 26,
respectively.
2.4.2. Data collection and handling
In case of 1 data collection strategy analysis as well as diffraction data processing were
performed automatically with CrysAlisPro (data collection, cell refinement and data
reduction). Initial unit cell parameters of 2–6 were obtained by least-squares refinement of
the xyz centroids of strong to medium strong reflections harvested from a series of 12 frames
in each of three orthogonally related regions of the reciprocal space using the evaluate routine
13

of the APEX software suite [95]. Final unit cell parameters were calculated at the end of
intensity measurements from xyz centroids of up to 10000 well-centred intense reflections of
the complete data set. Intensity data were collected via ω- and φ-scans in a range up to 2Θ =
56° with scan widths of 0.5° and scan speeds of 3 - 10 s/frame at a crystal to detector distance
of 40 mm. Collecting strategies were optimized by use of the collect routine of the APEX
software suite in order to reach an average data redundancy of 10 or better in about 24 h.
Information about crystal mosaicity as well as its scattering behaviour at higher Θ values was
derived from prescans for unit cell determinations. Integrated intensities were obtained with
the Bruker SAINT [96] software package using a narrow-frame algorithm performing spatial
corrections of frames, background subtractions, Lorenz and polarization corrections, profile
fittings and error analyses. Semi-empirical absorption corrections based on equivalent
reflections were made by use of the program SADABS [97]. Details on the data collection
2
parameters applied on the individual crystals are summarized in Table 1 with Rint = Σ|Fo -
2
2
2
2
Fo (mean)|/Σ[Fo ] and Rsigma = Σ[(Fo )]/Σ[Fo ]
Space groups [98] were determined from systematic absences and E-value statistics evaluated
by the examine data routine of the APEX program suite.
2.4.3. General information on structure solution and refinement
Structures were solved by direct methods and subsequent difference Fourier syntheses of the
program SHELXS [99] and refined by full-matrix least-squares techniques on F2 with
SHELXL [99], applying anisotropic displacement factors for all non-hydrogen atoms. Atomic
scattering factors were taken from International Tables for Crystallography [100].
Final agreement indices were defined as following: R1 = Σ||Fo| - |Fc||/Σ|Fo| and wR2 =
2
2 2
2
[Σ[w(Fo -Fc2)2]/Σ(Fo ) ]]1/2. Weighting function used: w = 1/[2(Fo ) + (pP)2 + qP] with P =
2
2
(Fo + 2Fc2)/3. Goof = [Σ[w(Fo -Fc2)2]/(n-p]1/2 where n is the number of reflections and p is
the total number of parameters refined. All non-hydrogen atoms were refined with
14

anisotropic displacement parameters and hydrogen atoms with common isotropic
displacement parameters for chemically related groups of hydrogen atoms.
Although most of the hydrogen atoms could be localised in difference Fourier syntheses,
those of the organic groups were refined in geometrically optimized positions riding on the
corresponding carbon atoms with C-H distances of 0.98 Å (-CH3), 0.99 Å (-CH2-) 1.00 Å (-
CH=) and 0.95 Å (CHarom). Hydrogen atoms bonded to oxygen were refined with respect to a
common O-H distance of 0.96 Å before they were fixed and allowed to ride on the
corresponding oxygen atoms. Further details on the results of structure refinement are
summarized in Table 1.
Disorder only was observed in case of the ethanol molecule of 5 as a whole with site
occupancies of 0.816/0.184. Bond lengths between the carbon atoms as well as between
carbon and oxygen were refined with bond constrains while the anisotropic displacement
parameters were set equal for neighbouring atoms.
Figures were drawn using DIAMOND [101] and Mercury [102], respectively. In the ball-and
stick models, all atoms are drawn as thermal displacement ellipsoids of the 40% level with
exception of the hydrogen atoms which are shown as spheres of arbitrary radii. Hydrogen
bonds are drawn in red as dashed sticks.
15

Table 1 Crystal data and structure refinement
Complex
Empirical formula C22H24MoN2O7
1
2
3
4
5
6
C21H20MoN2O7S
C20H17BrMoN2 O6S C24H20MoN2O6S
C21H20MoN2O6
C22H22MoN2O7S
Formula weight
Temperature
524.37
540.39
589.27
560.42
492.33
554.42
100(2) K
1.54184
100(2) K
0.71073
100(2) K
0.71073
100(2) K
0.71073
200(2) K
0.71073
100(2) K
0.71073
Wavelength [Å]
Crystal system,
space group
Monoclinic, P21/n Monoclinic, P21/c
Monoclinic, P21/c Monoclinic, P21/c
Triclinic, P
Triclinic, P
Unit cell
dimensions
a [Å]
16.3339(3)
7.9180(1)
18.8705(4)
90
10.8937(3)
12.3965(3)
15.7762(6)
90
8.9569(4)
10.9615(5)
11.7760(5)
91.519(2)
108.090(2)
111.060(2)
1013.12(8)
8.1458(5)
7.9829(3)
15.7158(8)
16.5012(8)
90
7.9379(4)
11.8274(5)
23.9551(8)
90
b [Å]
11.7962(6)
12.3553(6)
98.979(2)
c [Å]
α [°]
ß [°]
115.598(3)
90
101.033(2)
90
102.085(2)
105.001(1)
1093.20(10)
100.169(2)
90
96.283(2)
90
 [°]
Volume [Å3]
2201.01(8)
2091.10(11)
2037.68(16)
2235.51(16)
Z, Z', Calculated
density [g/cm3]
4, 1, 1.582
4, 1, 1.716
2, 1, 1.932
2, 1, 1.703
4, 1, 1.605
4, 1, 1.647
Absorption
coefficient [mm-1]
F(000)
5,283
0.775
2,765
0.742
0.685
1000
0.727
1128
1072
1096
584
568
Crystal size [mm]
0.1 x 0.08 x 0.05
0.15 x 0.10 x 0.06
0.34 x 0.23 x 0.06
0.35 x 0.25 x 0.22
0.49 x 0.20 x 0.13 0.27 x 0.22 x
0.07
Theta range for
data collection [°]
Reflections
3.002 to 74.014
21232
2.63 to 28.00
50824
3.50 to 28.00
78032
2.79 to 28.00
65582
2.59 to 28.00
2.94 to 28.00
68347
71048
collected
Reflections
unique, Rint
4397, 0.0166
99.9%
5044, 0.0965
99.9%
4887, 0.0517
99.8%
5280, 0.0675
99.8%
4863, 0.0767
98.7%
5295, 0.0594
98.0%
Completeness to
theta = 67.684
Max./min.
transmission
Data / restraints /
parameters
0.803/0.59479
4397 / 0 / 300
0.9521/0.8919
5044 / 0 / 295
0.868
0.8429/0.4577
4887 / 0 / 286
1,075
0.8514/0.7829
5280 / 0 / 313
1,097
0.9150/0.7293
4863 / 4 / 286
1,096
0.9536/0.8256
5295 / 0 / 307
1,143
Goodness-of-fit on 1,078
F2
R1, wR2
R1, wR2
0.0239, 0.0636
0.0449, 0.0834
0.0265, 0.0655
0.0225, 0.0589
0.0314, 0.0811
0.0433, 0.0949
0.0239, 0.0637
n/a
0.0738, 0.0917
0.0031(3)
0.0315, 0.0683
0.0064(6)
0.0238, 0.0595
0.0048 (6)
0.0393, 0.0889
n/a
0.0562, 0.0996
0.0011 (2)
Extinction
coefficient
Largest diff. peak / 0.360/-0.799
0.839/-0.741
1543188
0.571/-0.483
1543189
0.573/-0.564
1543190
0.715/-0.913
1543191
0.743/-0.863
1543192
hole e.Å-3
CCDC-number
1528262
16

2.5 Catalytic reactions
2.5.1. Oxidation of benzoin
For this oxidation reaction, benzoin (1.06 g, 5 mmol), aqueous 30% H2O2 (1.71 g, 15 mmol)
and catalyst (0.0005 g) were taken in 10 mL of methanol. The reaction was carried out at
refluxing temperature for 4 h. The progress of the reaction was examined by withdrawing
small aliquots at different time intervals and samples were extracted with n-hexane and then
analysing them quantitatively by gas chromatography. The effect of various parameters such
as amount of catalyst, amount of oxidant, and solvent were studied to optimize the reaction
conditions.
2.5.2. Oxidative bromination of salicylaldehyde
Dioxidomolybdenum complexes are also checked towards oxidative bromination of
salicylaldehyde. In a typical reaction, 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). The catalyst (0.0010 g) and 70% HClO4 (0.536 g, 3.75 mmol) were
added, and the reaction mixture was stirred at room temperature. Three additional 3.75 mmol
portions of 70% HClO4 were further added to the reaction mixture in three equal portions in
30 minutes intervals under continuous stirring. After 2 h, the white products have separated
out which was extracted with CH2Cl2 and dried. The crude mass was dissolved in methanol
and this material was subjected to gas chromatography, and the identity of the products
confirmed by GC–MS.
17

3. Results and Discussion
3.1. Synthesis
In the present study, seven dioxidomolybdenum(VI) complexes have been prepared from
various aroylazine ligands containing a bulky 3-hydroxy-2-naphthoic substituent (Scheme 1)
in order to observe their influence, if any, on the catalytic properties of the complexes.
Reaction of the aroylazines with MoO2(acac)2 in ethanol yielded orange or yellow crystalline
residue in good yield. Single crystals of 1 and 5 were obtained directly from the slow
evaporation of the filtrate of the reaction mixture, while complexes 24 and 6 were obtained
by recrystallizing the residue obtained in DMSO. Complexes (1–7) were completely soluble
in DMF, DMSO and partially soluble in CH3CN, ethanol and methanol. Magnetic
susceptibility and molar conductivity data indicate that all the complexes (17) are
diamagnetic and electrically non-conducting in solution. The detailed characterization of all
the complexes has been discussed in the respective section (IR, UV-Vis, NMR, ESI-MS and
X-Ray crystallography).
18

OH
O
O
R1
H
N
R1
R5
H
R5
MoO2(acac)2
N
R2
R3
R2
R3
N
N
EtOH / DMSO
O
O
O
HO
Mo
R4
X
R4
H2L1-7
O
[MoO2L1-7(X)]
Complex
Ligand
R1
R4
X
R5
R2
R3
[MoO2L1(EtOH)].EtOH (1)
[MoO2L2(DMSO)] (2)
H2L1
H
H
H
-
-
EtOH
H
H2L2
H2L3
H
H
H
H
H
OCH3
H
DMSO
DMSO
[MoO2L3(DMSO)]
(3)
Br
-
H
H2L4
H2L5
H2L6
H2L7
(4)
H
H
Ph
DMSO
EtOH
H
[MoO2L4(DMSO)]
(5)
H
[MoO2L5(EtOH)]
CH3
CH3
H
H
H
H
H
-
OCH3
DMSO
DMSO
[MoO2L6(DMSO)] (6)
-
-
[MoO2L7(DMSO)]
(7)
OCH3
H
H
Scheme 1. Synthesis of the dioxidomolybdenum(VI) complexes [MoO2L1(X)].X (1) and
[MoO2L27(X)] (2–7).
3.2. Spectral Characteristics
Spectral characteristics of all the ligands (H2L1–7) and complexes (1–7) are given in the
Experimental Section. All the ligands exhibit two bands in the range of 33873556 cm-1 in
the FTIR spectra due to the presence of two OH groups [32,103,104]. The stretching for the
NH group is found the region between 30033008 cm-1 and the C=O stretching frequency
is obtained in the range of 16421649 cm-1 [32,103,104]. The disappearance of NH, C=O
and one of the OH stretching bands in the IR spectra of the complexes indicate the
formation of metal complexes. The sharp peak at 15871615 cm-1 is probably due to the
C=NN=C moiety in the complex formed [32,103,104]. The presence of two strong peaks
in the range 902966 cm-1 is due to the Mo=O stretching [32,103,104] which indicates the
presence of dioxido group in the complexes.
19

Electronic spectra of the complexes were recorded in DMSO. All the complexes exhibit
medium intensity bands in the range 382428 nm assignable to ligand to metal charge
transfer (LMCT) and strong intensity bands within 259341 nm which may be due to ligand
centered transitions [32,103,104]. The representative absorbance spectrum of 6 is given in
Fig. 1.
4000
259
3000
332
2000
400
1000
0
300
400
Wavelength / nm
Fig. 1. Electronic absorption spectrum of 6 (1.5 × 10−4 M) in DMSO.
13
1H and C NMR of all the compounds were recorded in DMSO-d6 and the data are given in
the experimental section. The coordinating modes of H2L1–7 were confirmed by comparing
1
1
their H NMR spectral patterns with those of the corresponding complexes. The H NMR
1
spectra of all the ligands (H2L1-7) and complexes (1–7) are given in Figs. S1–S14. The H
NMR spectrum of the free ligands exhibits resonance in the range δ = 13.55–11.94 ppm due
to NH group [32,103,104]. Two peaks in the range of δ = 12.25–10.92 ppm were observed
due phenolic OH groups, while the singlet at δ = 9.57–8.21 ppm was due to the azomethine
–CH proton [32,103,104]. The aromatic protons from ligands are clearly observed in the
expected region δ = 8.53–6.49 ppm. Singlets for the OCH3 protons of H2L2, H2L6and H2L7
were observed at around 3.78 ppm, while singlets for the CH3 protons of H2L5 and H2L6
were observed at 3.36 and 2.43 ppm respectively. In the NMR spectra of complexes, one of
the peak for the aromatic OH proton disappeared due to the deprotonation of phenolic
20

group. The aromatic protons of the complexes were observed in the range δ = 8.64–6.58 ppm.
13
In the C NMR spectra of the ligands H2L1–5, spectral signals for the aromatic carbons are
found in the downfield region in the range δ = 169.95102.06 ppm [32,103,104]. The signals
for the aliphatic carbons of for –OCH3 and –CH3 of H2L2, H2L5, H2L6 and H2L7 appeared in
13
the range of δ = 55.9313.60 ppm. While in the C NMR spectra of complexes 17, signals
for the aromatic carbons are found in the downfield region in the range δ = 169.65111.42
ppm [32,103,104]. The 13C NMR spectra of all the ligands (H2L1-7) and complexes (1–7) are
given in Figs. S15–S28.
3.3. ESI-MS
ESI-MS spectra of 17 have been recorded in CH3CN. The characteristic molecular ion peak
for 17 appear at 524.20 [M]+, 540.21 [M]+, 589.50 [M]+, 560.15 [M]+, 492.02[M]+, 554.02
[M]+, and 540.85 [M]+. The representative ESI-MS of 7 is given in Fig. S29.
3.4. Electrochemical properties
Electrochemical properties of the complexes have been studied by cyclic voltammetry in
CH3CN solution (0.1 M TBAP). Voltammetric data are given in Table 2 and a representative
voltammogram of 6 is given in Fig. 2. The CV traces of all the complexes are similar and
exhibit two irreversible reductive responses within the potential window 0.50 to 1.36 V,
which are assigned to MoVI/MoV and MoV/MoIV processes respectively [89]. Two oxidation
waves at positive potentials in the range of +1.25 to +1.69 V are assigned to the oxidation of
the coordinated ligand [105]. The representative cyclic voltammogram of 6 showing the
oxidation potentials is given in Fig. S30. Single electron processes were verified by
comparing the current height with that of the standard ferrocene–ferrocenium couple under
identical experimental conditions.
21

Table 2 Cyclic voltammetric results for oxidomolybdenum(VI) complexes (1  7) at 298 K
Complexes
Epc [V][a]
–0.51, –1.34
–0.54, –1.30
–0.50, –1.31
–0.52, –1.35
–0.52, –1.36
–0.51, –1.32
–0.55, –1.36
1
2
3
4
5
6
7
[a]Solvent: CH3CN; 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.
Fig. 2. Cyclic voltammogram of 6 in CH3CN; scan rate: 50 mV/s and potentials recorded vs
Ag/AgCl.
22

3.5. X-ray structure description
The solid state structures of complexes 1−6 are shown in Fig. 3 and the selected bond lengths
and bond angles are listed in Table 3.
Table 3 Selected bond lengths [Å] and angles (°) for complexes 16
Complexes
Bond lengths
O(1)-C(1)
1
2
3
4
5
6
1.319(2) 1.322(4) 1.319(2) 1.326(2) 1.310(2) 1.309(4)
1.350(2) 1.365(3) 1.345(3) 1.341(2) 1.351(3) 1.351(4)
2.236(1) 2.240(3) 2.256(2) 2.227(1) 2.261(2) 2.253(3)
1.690(1) 1.694(2) 1.698(2) 1.701(1) 1.683(2) 1.698(2)
1.718(1) 1.718(2) 1.700(2) 1.713(1) 1.698(2) 1.695(2)
1.925(1) 1.919(2) 1.928(2) 1.934(1) 1.908(2) 1.922(2)
2.007(1) 2.036(2) 1.996(2) 2.012(1) 2.018(2) 2.009(2)
2.236(1) 2.240(3) 2.256(2) 2.227(1) 2.261(2) 2.253(3)
O(2)-C(12)
Mo(1)-N(1)
Mo(1)-O(3)
Mo(1)-O(4)
Mo(1)-O(2)
Mo(1)-O(1)
Mo(1)-N(1)
Mo(1)-O(solvent) 2.292(1) 2.347(2) 2.309(2) 2.322(1) 2.329(4) 2.270(2)
Bond angles
O(3)-Mo(1)-O(4) 105.45(6) 105.7(1) 105.58(8) 105.59(6) 105.29(9) 105.0(1)
O(3)-Mo(1)-O(2) 99.26(6) 99.5(1) 99.46(7) 100.11(5) 99.90(9) 100.2(1)
O(4)-Mo(1)-O(2) 103.54(6) 104.0(1) 103.01(7) 102.49(5) 103.14(9) 103.5(1)
O(3)-Mo(1)-O(1) 97.98(6) 95.0(1) 96.17(7) 96.95(5) 97.91(9) 96.6(1)
O(4)-Mo(1)-O(1) 96.67(5) 96.5(1) 96.78(7) 97.94(5) 97.20(8) 97.1(1)
O(2)-Mo(1)-O(1) 148.75(5) 150.5(1) 150.36(6) 148.66(5) 148.21(7) 149.0(1)
O(3)-Mo(1)-N(1) 93.81(6) 94.3(1) 93.55(7) 94.11(5) 95.40(8) 90.7(1)
O(4)-Mo(1)-N(1) 158.99(6) 158.0(1) 158.93(7) 159.20(5) 158.13(8) 162.4(1)
O(2)-Mo(1)-N(1) 81.07(5) 81.3(1) 82.00(6) 80.06(5) 79.72(7) 81.1(1)
O(1)-Mo(1)-N(1) 71.94(5) 72.1(1) 71.99(6) 72.62(4) 72.57(6) 72.76(1)
O(3)-Mo(1)-
O(solvent)
169.66(6) 167.8(1) 167.42(7) 169.83(5) 172.80(9) 166.7(1)
23

(a)
(b)
(d)
(c)
(f)
(e)
Fig. 3. Ball-and-stick models of complexes with the atomic numbering scheme used [Fig.
3(a): 1; Fig. 3(b): 2; Fig. 3(c): 3; Fig. 3(d): 4; Fig. 3(e): 5; Fig. 3(f): 6] with exception of the
hydrogen atoms, which are drawn as spheres of arbitrary radius, all other atoms are
represented as thermal displacement ellipsoids of 50% probability level.
24

The asymmetric unit of all these complexes consists of a complete formula unit. All
complexes exhibit the same structural features consisting of a central molybdenum atom
coordinated by two double bonded oxygen atoms, the tridentate organic ligand and an
additional solvent molecule like dimethyl sulfoxide (2, 3, 4, 6) or ethanol (1, 5) acting as
Lewis Base. In case of 1, a second ethanol molecule is hydrogen bonded to the ethanol
molecule coordinating the Mo atom. Conformational differences within the complexes arise
from steric and electronic requirements of the individual ligands. At the metal site, the MoO2-
fragment with its double bonded oxygen atoms is only slightly affected by ligand effects. The
Mo=O bonds are in a very narrow range [1.690(1) – 1.718(2) Å, mean value: 1.701(11) Å] as
are the O=Mo=O bond angles [105.0(1)°-105.7(1)°, mean value: 105.4(3)°]. In each case, the
organic ligand coordinates the Mo atom via two oxygen atoms and one nitrogen atom.
Significant bond length variations are observed in case of the corresponding Mo-O bonds.
Mo-O bonds of the carbonyl group [O(1)] are slightly weaker [1.996(2) – 2.036(2) Å, mean
value: 2.013(13) Å] as those [1.908(2) -1.934(1) Å, mean value: 1.923(9) Å] of the alkoxide
group [O(2)]. Conversely, the corresponding C-O bonds of the carbonyl group are shorter
[1.309(4) – 1.326(2) Å, mean value: 1.318(7) Å] as those of the alkoxide group [1.345(3) –
1.365(3) Å, mean value: 1.351(8) Å]. Both types of coordinative Mo-O bonds are shorter
than the coordinative Mo-N bonds [N(1), 2.227(3) – 2.261(3) Å, mean value 2.246(13) Å].
As a result of the ligand coordination to the Mo atom a 5-membered, and a six-membered
chelate ring are formed. In all compounds, the oxygen atom of the solvent molecule (DMSO,
EtOH) acting as Lewis Base to the central Mo atom gives rise to the weakest bond [2.270(2) -
2.347(2) Å, mean value: 2.312(28) Å] in the complex. Lability of this bond is expressed by
the broad range these Mo-O bond cover, as well as by the relatively large bond angles
[166.7(1)° – 172.80(9)°, mean value 169.0(22)°] between the solvent O atoms and their trans
positioned, double bonded O(3) atoms.
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An additional common feature of all compounds is the presence of an intramolecular
hydrogen bond between the OH group [O(5)-H(5)] at C(3) and the non-coordinating nitrogen
atom [N(2)], giving rise to a six-membered, nearly planar ring (Fig. 4). These hydrogen
bonds are characterized by O…N distances and O-H…N angles of 2.538(2) – 2.630(2) Å, and
145.6° - 153.8°, respectively.
Fig. 4. View on the coordination sphere of the Mo atom and intramolecular O-H…N
hydrogen bond. With exception of the hydrogen atoms, which are drawn as spheres of
arbitrary radius, all other atoms are represented as thermal displacement ellipsoids of 50%
probability level. For clarity, only the backbone of these structural features is drawn.
26

3.6. Catalytic activity studies
3.6.1. Oxidation of benzoin
The oxidation of benzoin is one of the most significant organic transformations in chemistry.
Benzil, one of the oxidized products of benzoin, is a very functional intermediate for the
synthesis of heterocyclic compounds and benzylic acid rearrangements [106]. The
molybdenum complexes successfully catalyze the oxidation of benzoin using 30% aqueous
H2O2 as an oxidant. Scheme 2 shows the main products obtained from this reaction.
CH3 CH3
O
O
O
O
O
Catalyst
OH
OH
O
H2O2/MeOH
c
a
b
Scheme 2.Various oxidation products of benzoin. (a) benzil, (b) benzaldehyde-dimethyl
acetal and (c) benzoic acid.
[MoO2L1(EtOH)].EtOH (1) was considered as a representative catalyst to optimize the
reaction conditions for the maximum oxidation of benzoin. The effect of oxidant was studied
by considering the oxidant to substrate ratios of 1 : 1, 2 : 1 and 3 : 1 for the fixed amount of
catalyst (0.0010 g) and substrate (1.06 g, 5 mmol) in 10 mL of refluxing methanol. As shown
in Fig. 5(a) and entry no. 3 of Table 4, a maximum of 95 % conversion of benzoin was
achieved at the oxidant to substrate ratio of 3 : 1, after 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 two different amounts of [MoO2L1(EtOH)].EtOH (1) viz.
0.0005 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 reaction was monitored at reflux temperature of
27

methanol. Fig. 5(b) shows that maximum of 94 % conversion was achieved with 0.0005 g of
catalyst. This conversion improved only marginally to 98% with 0.0015 g of catalyst. Thus
only 0.0005 g of catalyst can be considered sufficient to optimize other reaction conditions.
The amount of solvent also influences on the oxidation of benzoin. It was concluded (Fig.
5(c) and entry no. 6 and 7 of Table 4) that 10 mL methanol was sufficient to obtain
maximum conversion under above optimized reaction conditions. Table 4 summarizes
different experimental conditions for the oxidation of benzoin. Thus, from these experiments,
the best reaction conditions for the maximum oxidation of benzoin as concluded are: catalyst
[MoO2L1(EtOH)].EtOH (1) (0.0005 g), benzoin (1.06 g, 5 mmol) and 30% H2O2 (1.7 g, 15
mmol) and refluxing methanol (10 mL).
Fig. 5(d) exhibits the selectivity of products along with the conversion 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), [MoO2L1(EtOH)].EtOH (1) (0.0005
g) and methanol (10 mL) under reflux condition. It is clear from the plot that all products
form with the conversion of benzoin. The highest selectivity of benzil (ca. 73.7%) was
observed in the first one hour. With intervene of time, its selectivity slowly decreases and
finally becomes almost constant and reaches 45 % after 4h. In the case of benzoic acid ca.
26.3% selectivity was found in first one hour which increases in next two hour and further
decrease and reaches 39.4% in last hour. The selectivity of benzaldehyde-dimethylacetal
increases continuously from 2.5 to 15.6 %. Thus, with the maximum benzoin oxidation of 94
% after 4 h of reaction time, the selectivity of the reaction products varies in the order:
benzil(45%) > benzoic acid (39.4 %) > benzaldehyde-dimethylacetal (15.6 %).
Other catalysts were also tested under these reaction conditions and gave similar results.
Table 5 provides turnover frequency (TOF) and selectivity details of products. It is clear
28

from the table that other complexes are also catalytically active and show equally good
results with very high turnover frequency and the selectivity order of various products.
In one of our previous reports, where oxidomolybdenum(VI) complexes of similar ligands
were employed as catalysts [32], it was found that during the catalytic oxidation of benzoin, a
small amount of methylbenzoate was formed along with benzil, benzoic acid and
benzaldehyde-dimethylacetal. Similar observations were also reported by Maurya et al. where
dioxidomolybdenum(VI) complexes of tribasic pentadentate Schiff base ligands were used as
catalysts [60]. In the present case, a bulky 3-hydroxy-2-naphthoic hydrazide was incorporated
in the ligands of the reported oxidomolybdenum(VI) complexes (17). Interestingly when
17 was employed as catalysts, the formation of methylbenzoate could be avoided. Also the
selectivity of formation of benzil now increased to 45% which was 1516% in the previous
reports. Kurapati et al. [63] have reported the catalysis of oxidation of benzoin with
oxidomolybdenum(VI) complexes containing unsymmetrical tripodal NO3 donor ligands
where similar product selectivity was observed. However, it was reported that selectivity of
formation of benzil was around 30% and the TOF was 125 h-1. In the present case, a better
selectivity of benzil (45%) and TOF (11621468 h-1) was achieved.
29