Synthesis and Characterization of Glucose Derived Dioxo-molybdenum (VI) Complexes and Their Application in Sulphide Oxidation

Article abstract of DOI:10.1007/s10562-014-1463-6

Three new molybdenum (VI) complexes of 4,6-O-ethylidene-β-D-glucopyranosylamine derived ligands has been synthesized and the same has been used in the oxidation of thioanisole along with an earlier reported analogous complex. A selective oxidation of thio

Full text of DOI:10.1007/s10562-014-1463-6

Catal Lett
DOI 10.1007/s10562-014-1463-6
Synthesis and Characterization of Glucose Derived Dioxo-
molybdenum (VI) Complexes and Their Application in Sulphide
Oxidation
•
Ajay K. Sah Noorullah Baig
Received: 14 September 2014 / Accepted: 10 December 2014
Ó Springer Science+Business Media New York 2015
Abstract Three new molybdenum (VI) complexes of 4,6-
O-ethylidene-b-D-glucopyranosylamine derived ligands
has been synthesized and the same has been used in the
oxidation of thioanisole along with an earlier reported
analogous complex. A selective oxidation of thioanisole to
methyl phenyl sulphoxide in high yield is achieved using
1:1 mixture of thioanisole and urea hydrogen peroxide
(UHP) in ethanol. A longer reaction time or excess of UHP,
leads to the formation of corresponding sulphone, which
was confirmed using HPLC and NMR measurements.
Graphical Abstract The oxidation of thioanisole into
corresponding sulphoxide and sulphone has been explored
using dioxo-molybdenum (VI) complexes of 4,6-O-ethyli-
dene-b-D-glucopyranosylamine derived Schiff base
ligands.
1 Introduction
Biochemistry is rich with the chemistry of lighter elements
(at. no.\35) [1, 2], with few exceptions and especially that
for group 6 elements of the periodic table. The lightest
element of this group (Cr) is mainly regarded for its tox-
icity inside the biological system, while tungsten (W) is
found generally in thermophilic organisms [3]. Mo controls
several biochemical reactions in the form of nitrogenase,
nitrate reductase, DMSO reductase, xanthine oxidase etc.,
[4, 5]. The catalytic reactions of Mo in various molybdo-
enzymes inspires the inorganic chemists to explore the
catalytic reactions using the complexes containing this
metal ion due to its adaptable nature and accessible stable
oxidation states [6]. Molybdenum complexes have been
used in industrial ammoxidation of olefins [7], olefin
epoxidation [8] as well as olefin metathesis [9].
O
S
Structurally characterized sugar containing molybde-
num complexes are known, since more than a decade,
however its use has only been explored by Zhao et al. in the
epoxidation of cyclooctene and cis-, trans-b-methylstyrene
[10]. Molybdenum complexes are well known for oxo
transfer reactions like isomerization of allyl alcohols,
epoxidation, phosphine oxidation, sulfide oxidation, oxi-
dative bromination etc. [11–14], however utility of sugar
containing Mo complexes are scarce. Lack of utility of
sugar derived molybdenum complexes prompted us to
perform the catalytic reaction using these complexes and
along this line we have explored the sulfide oxidation
reactions using four glucose derived molybdenum com-
plexes. Synthesis of two ligands [15, 16] and one molyb-
denum complex [17] along with their crystal structures
have already been reported and here we are presenting the
synthesis and characterization of new two ligands
(Scheme 1; H3L3, H3L4) and molybdenum complexes of
1 e
q.
UH
P
Catalyst
(Mo(VI) complex)
S
EtOH
rt
O
O
S
5 e
q.U
HP
Keywords Glucopyranosylamine Á Molybdenum Á Urea
hydrogen peroxide Á Sulphoxide Á Sulphone
A. K. Sah (&) Á N. Baig
Department of Chemistry, Birla Institute of Technology and
Science, Pilani, Pilani Campus, Rajasthan 333031, India
e-mail: asah@pilani.bits-pilani.ac.in; ajay_ksah@yahoo.com
123

A. K. Sah, N. Baig
Scheme 1 Synthetic route for
ligands
O
H
H
O
MeOH /Reflux
O
O
O
O
N
O
O
NH2
H
O
H
HO
C
O
R
HO
(H3L1),
R=2-Hydroxy-phenyl
2-Hydroxy-3-tert- butylphenyl (H3L2),
2-Hydroxy-3,5-Di-tert- butylphenyl (H3L3)
2-Hydroxy-napthyl (H3L4)
1H and 13C NMR were recorded in DMSO-d6 on a
Bruker Avance spectrometer and data was processed using
MestReNova software. The solution electronic spectra
were recorded on Shimadzu UV-260 spectrophotometer
and IR spectra on ABB Bomen MB 3000 FTIR machine
using KBr Matrix. HRMS of organic samples and ESI-MS
of complexes were recorded on Thermoscientific Q Exac-
tive and Hewlett-Packard’ HP GS/MS 5890/5972 mass
spectrometer respectively.
Ligand
Complex No.
O
O
N
O
H3L1
H3L2
H3L3
H3L4
1
2
3
4
O
Mo
OH2
O
OH
O
O
Fig. 1 Molybdenum complexes of H3Ln
2.2 Preparation of N-(3,5-di-tert-butyl-2-
hydroxybenzilidene)-4,6-O-ethylidene-b-D-
glucopyranosylamine (H3L3)
H3Ln (n = 2–4; Fig. 1). The sulfide oxidation reaction has
been optimized using 4,6-O-ethylidene-N-(2-hydroxyben-
zylidene)-b-D-glucopyranosylamine derived molybdenum
(VI) [Mo (VI)] complex and reactions of rest three
molybdenum complexes were performed under optimized
condition. We have achieved an isolated yield of 89 %
methyl phenyl sulfoxide using (1:1) urea hydrogen perox-
ide (UHP) in 15 min. Longer reaction time and excess of
UHP concentration leads to the conversion of sulphoxide to
sulphone. Formation of both sulphoxide and sulphone have
been confirmed by HPLC and NMR spectroscopy.
This compound was synthesiszed adapting the procedure
reported for H3L2 [16], but using 4,6-O-ethylidene-b-D-
glucopyranosylamine (2.00 g, 9.75 mmol) in ethanol
(20 mL), and 3,5-di-tert-butyl-2-hydroxybenzaldehyde
(2.510 g, 10.7 mmol). Yield: 3.301 g (80 %); yellow solid;
mp 106–108 °C; IR (KBr; cm-1): 3433, 2955,1628, 1103.
UV–Vis [kmax; nm (e; L cm-1 mol-1) in DMSO]: 262
1
(320000), 333 (100000). H NMR (DMSO-d6 400 MHz,
ppm): d 13.64 (1H, s, Ar–OH), 8.59 (1H, s, HC=N), 7.35
(2H, s, ArH), 5.56 (1H, d, J = 6.0 Hz, glucose–OH), 5.37
(1H, d, J = 5.6 Hz, glucose–OH), 4.77 (1H, q, J = 4.8 Hz,
ethylidene CH), 4.54 (1H, d, J = 8.4 Hz, glucose H-1),
4.06 (1H, m, glucose H-5), 3.57–3.40 (3H, m, glucose H-3,
H-4, H-6a), 3.28 (1H, m, glucose H-6b), 3.13 (1H, m,
In conclusion, this paper deals with the synthesis and
characterization of glucose derived ligands, their Mo (VI)
complexes and application of latter in sulfide oxidation
reactions.
t
t
glucose H-2), 1.39 (9H, s, Bu),1.27 (12H, m, Bu and
ethylidene CH3); 13C NMR (101 MHz, DMSO-d6, ppm): d
167.22 (C=N), 157.92, 140.35, 136.10, 127.64, 127.32,
117.84, 99.09, 95.75, 80.60, 75.22, 73.73, 68.31, 67.80,
35.03, 34.33, 31.74, 29.70, 20.79; HRMS: m/z calcd for
(M ? H)? C23H36NO6 422.2543; found 422.2754.
2 Experimental
2.1 General
All experiments were performed under normal atmospheric
conditions and room temperature. 2-Hydroxy-1-napthal-
dehyde and UHP were purchased from Sigma-Aldrich
India, 3,5-di-tert-butyl-2-hydroxy benzaldehyde and Mo
(VI) oxide bis(2,4-pentanedionate) [MoO2(acac)2] were
purchased from Alfa Aeser. The compounds H3L1 and
H3L2 were synthesized following the reported literature
procedure [15, 16]. The rest of the chemicals along with
solvents were procured from local suppliers and solvents
were purified and dried following the standard methods
before use.
2.3 Preparation of N-(2-hydroxynapthylidene)-4,6-O-
ethylidene-b-D-glucopyranosylamine (H3L4)
This compound was synthesized following the procedure
reported for H3L1 [15], but using 4,6-O-ethylidene-b-D-
glucopyranosylamine (0.102 g, 0.5 mmol) and 2-hydroxy-
1-napthaldehyde (0.094 g, 0.55 mmol) in ethanol (3 mL).
Yield: 0.153 g (87 %), fluoresent yellow solid; mp: chared
at 228–230 °C; IR (KBr; cm-1): 3433, 3209,1643, 1095;
123

Synthesis and Characterization of Glucose
UV–Vis [kmax; nm (e; Lcm-1mol-1) in DMSO]: 304
(6080), 365 (2740), 403 (3820), 422 (3700). 1H NMR
(DMSO-d6 400 MHz, ppm): d 14.30 (1H, m, Ar–OH), 9.25
(1H, d, J = 7.2 Hz, HC=N), 8.13 (1H, d, J = 8.8 Hz, Ar–
H), 7.85 (1H, d, J = 9.6 Hz, Ar–H), 7.73 (1H, d,
J = 8.0 Hz, Ar–H), 7.50 (1H, m, Ar–H), 7.29 (1H, t,
J = 7.2 Hz, Ar–H), 6.86 (1H, d, J = 9.2 Hz, Ar–H), 5.76
(1H, d, J = 6.0 Hz, glucose–OH), 5.47 (1H, d, J = 5.2 Hz,
glucose–OH), 4.78 (2H, m, ethylidene CH and glucose
H-1), 4.08 (1H, m, glucose H-5), 3.57–3.39 (3H, m, glu-
cose H-3, H-4, H-6a), 3.32–3.22 (2H, m, glucose H-2,
H-6b), 1.25 (3H, d, J = 5.2 Hz, ethylidene CH3); 13C
NMR (101 MHz, DMSO-d6, ppm): d 173.88 (C=N),
159.77, 137.73, 134.04, 129.48, 128.66, 126.55, 123.87,
123.54, 119.54, 107.22, 99.11, 92.49, 80.32, 74.90, 73.69,
68.72, 67.64, 20.76; HRMS: m/z calcd for (M ? H)?
C19H22NO6 360.1447; found 360.1593.
(KBr; cm-1): 3379, 2955, 1643, 1103, 903. UV–Vis [kmax
;
nm (e; Lcm-1mol-1) in DMSO]: 282 (879000), 359
1
(198000). H NMR (DMSO-d6 400 MHz, ppm): d 8.52
(1H, d, J = 2.4 Hz, HC=N), 7.59 (1H, d, J = 2.8 Hz,
ArH), 7.47 (1H, d, J = 2.4, ArH), 5.56 (1H, d, J = 5.6 Hz,
glucose–OH), 4.76 (1H, q, J = 5.1 Hz, ethylidene CH),
4.67 (1H, dd, J = 8.8 Hz, 2.2 Hz, glucose H-1), 4.17 (1H,
m, glucose H-5), 3.70–3.62 (3H, m, glucose H-3, H-4,
H-6a), 3.49 (1H, m, glucose H-6b), 3.31 (1H, glucose H-2),
t
t
1.35 (9H, s, Bu),1.26 (12H, m, Bu and ethylidene CH3);
13C NMR (101 MHz, DMSO-d6, ppm): d 160.24 (C=N),
159.22, 141.59, 138.31, 129.69, 129.59, 121.00, 99.42,
91.30, 85.30, 81.27, 77.29, 73.61, 70.00, 67.78, 35.39,
34.48, 31.66, 30.04, 20.70; ESI-MS: m/z calcd for
(M ? H)? C23H35MoNO9 566.4; found 566.9.
2.6 Preparation of MoO2(HL4) (4)
2.4 Preparation of MoO2(HL2) (2)
This compound was prepared following the procedure
adopted for complex 1 [17] but using H3L4 (0.143 g,
0.4 mmol) and MoO2(acac)2 (0.097 g, 0.3 mmol) Yield:
0.120 g (80 %), flouresent yellow solid; mp:[ 250 °C; IR
To a methanolic solution (5 mL) of H3L2 (0.292 g,
0.8 mmol), MoO2(acac)2 (0.195 g, 0.6 mmol) was added
and the reaction mixture was stirred at room temperature
for 13 h to result in clear yellow solution. The reaction
mixture was concentrated, residue was dissolved in diethyl
ether (3 mL) and excess hexane was added to that while
stirring to result in light yellow solid product, which was
filtered and dried under vacuum.Yield: 0.234 g (76 %);
yellow solid; mp [250 °C; IR (KBr; cm-1): 3649, 3433,
3163, 2962, 1643, 1011, 910. UV–Vis [kmax; nm (e;
(KBr; cm-1): 3433, 1628, 1142, 1103, 895. UV–Vis [kmax
;
nm (e; L cm-1 mol-1) in DMSO]: 313 (1349000), 380
1
(525000). H NMR (DMSO-d6 400 MHz, ppm): d 9.30
(1H, d, J = 2.0 Hz, HC=N), 8.15 (1H, d, J = 8.4, ArH),
8.06 (1H, d, 9.2 Hz, ArH), 7.90 (1H, d, J = 7.6 Hz, ArH),
7.62 (1H, m, ArH), 7.43 (1H, t, J = 7.2 Hz, ArH), 7.16
(1H, d, J = 8.8 Hz, ArH), 5.62 (1H, d, J = 5.6 Hz, glu-
cose–OH), 4.77 (2H, m, glucose H-1 and ethylidene CH),
4.27 (1H, m, glucose H-5), 3.82–3.69 (3H, m, glucose H-3,
H-4, H-6a), 3.59 (1H, m, glucose H-6b), 3.37 (1H, glucose
H-2), 1.26 (3H, d, J = 4.8 Hz, ethylidene CH3); 13C NMR
(101 MHz, DMSO-d6, ppm): d 163.20 (C=N), 154.04,
136.56, 133.23, 129.47, 128.96, 128.32, 124.54, 121.99,
120.58, 111.93, 99.41, 91.48, 85.53, 81.27, 73.51, 70.02,
67.79, 20.70.
L cm-1 mol-1
)
in DMSO]: 276 (shoulder), 354.5
1
(355000). H NMR (DMSO-d6 400 MHz, ppm): d 8.51
(1H, d, J = 2.4 Hz, HC=N), 7.60 (1H, d, J = 8.8 Hz,
ArH), 7.48 (1H, dd, J = 7.6 Hz, 1.6 Hz, ArH), 6.91 (1H, t,
J = 7.6 Hz, ArH), 5.62 (1H, d, J = 5.6 Hz, glucose–OH),
4.78–4.73 (2H, m, glucose H-1, ethylidene CH), 4.19 (1H,
m, glucose H-5), 3.77–3.67 (3H, m, glucose, H-3, H-4,
H-6a), 3.54 (1H, m, glucose H-6b), 3.34 (1H, glucose H-2),
t
1.36 (9H, s, Bu), 1.27 (3H, d, J = 4.8 Hz, ethylidene
CH3); 13C NMR (101 MHz, DMSO-d6, ppm): d 161.24
(C=N), 159.81, 139.25, 133.34, 132.49, 121.52, 119.74,
99.41, 91.32, 85.37, 81.22, 73.59, 70.03, 67.76, 35.24,
29.98, 20.71; ESI-MS: m/z calcd for (M ? H)? C19H27
MoNO9 510.3; found 510.9.
2.7 General Procedure for Selective Oxidation
of Thioanisole to Methyl Phenyl Sulphoxide
To a mixture of molybdenum complexes (1–4; 0.05 mmol)
and thioanisole (1 mmol) in 3 mL of ethanol, UHP
(1 mmol) was added and the reaction mixture was stirred at
room temperature. The progress of reaction was monitored
by TLC as well as HPLC. The reaction mixture was con-
centrated under reduced pressure and the pure product was
isolated by column chromatography using silica gel as
solid support and a mixed solvent (n-hexane/ethyl acetate
70:30) as eluent.
2.5 Preparation of MoO2(HL3) (3)
This compound was prepared following the procedure
adopted for complex 2, but using (H3L3) (0.198 g,
0.47 mmol) and MoO2(acac)2 (0.150 g, 0.46 mmol).
Yield: 0.194 g (74 %); yellow solid; mp 223–225 °C; IR
123

A. K. Sah, N. Baig
Fig. 2 HPLC spectrum of the
reaction mixture set for
thioanisole oxidation; inset
represents the spectrum of
isolated pure methyl phenyl
sulphoxide after column
chromatography
Table 1 Summary of control reaction for the oxidation of thioanisole
Table 3 Results of methyl phenyl sulphoxide formation by different
catalysts under optimized reaction conditions
Entry Catalyst
Substrate UHP
Yield of
Yield of
1 (mmol) (mmol)
(mmol) sulphoxidea sulphonea
Entry
Catalyst
Solvent
Time (min)
Isolated yield (%)
(%)
(%)
1
2
3
4
1
2
3
4
Ethanol
Ethanol
Ethanol
Ethanol
15
15
15
15
89
83
80
75
1
2
3
4
5
0
1
1
1
1
1
1
0
1
2
5
0
0
0.05
0.05
0.05
0.05
0
0
89
20
0
0
50
94
The catalyst number has been represented in bold to make a differ-
ence with that of the entry serial number
Refer Fig. 1 for catalyst
a
Isolated yield after 15 min of reaction time
1643 cm-1. HRMS signals at m/z 422.2754 and 360.1593
corresponds to (M ? H)? ion peaks of H3L3 and H3L4
respectively. 1H and 13C NMR spectrum recorded in
DMSo-d6, mentioned in the experimental section, clearly
confirmed the formation of pure products H3L3 and H3L4.
Both the ligands exhibited a coupling constant for sac-
charide C1–H, 3JH–H = 8.3 Hz, supporting the presence of
b-anomeric form of the sugar moiety.
Table 2 Effect of solvent on oxidation of thioanisole to methyl
phenyl sulphoxide
Entry
Solvent
Time
Isolated yield (%)
1
2
3
4
5
6
Ethanol
Methanol
Acetonitrile
Toluene
THF
15 min
15 min
15 min
2 h
89
85
72
33
66
52
After confirming the ligands formation, H3L1, H3L2,
H3L3, and H3L4 were treated with MoO2(acac)2 in meth-
anol to afford the corresponding sugar bound Mo (VI)
complexes. FTIR studies of all the complexes exhibited
characteristic vibration bands in the range of 910–
895 cm-1, corresponding to mMo=O. No appreciable chan-
ges were observed for mC–O and mC=N stretching frequencies
of complexes with respect to the corresponding free
ligands, but a change in the mO–H region of spectra was
observed. Sharp bands in the OH region of the ligands
converted into broad ones after metalation reaction, indi-
2 h
DCM
2 h
3 Results and Discussion
3.1 Synthesis and Characterization of Ligands
and Complexes
1
cating the increment in hydrogen bonding interactions. H
Synthesis of H3L3 and H3L4 were carried out by con-
densing 4,6-O-ethylidene-b-D-glucopyranosylamine with
3,5-di-tert-butyl-2-hydroxybenzaldehyde and 2-hydroxy-1-
napthaldehyde respectively. The product formation was
confirmed by FTIR, UV–Visible, NMR and mass spec-
troscopy. FTIR spectra of H3L3 and H3L4 exhibited strong
characteristic bands for mC–O and mO–H in the range of
1157–1011, and 3433–3209 cm-1 respectively. mC=N
stretch for H3L3 appeared at 1628 and that for H3L4 at
NMR spectra of all the molybdenum complexes exhibit the
loss of phenolic, and C2-hydroxyl proton of the glucose
moiety revealing the binding of metal ion with the ligand
via deprotonation. In all the complexes, signal of glucose
C3–OH shifted down field with respect to the free ligands.
Complexation of ligands (H3L2, H3L3, H3L4) with Mo
(VI) also influenced the chemical shifts of the saccharide
skeletal protons and all such five protons underwent
downfield shift by about 0.2 ppm. Even in the molybdenum
123

Synthesis and Characterization of Glucose
complexes, the b-anomeric form of the ligands were
retained as JH–H were found to be 8.9.
using molybdenum complexes bearing ligand with ONO
chelating site. The advantage of our method is that we got
better yield in shorter time (15 min in compared to 30)
using similar catalytic loading. Moreover, we have per-
formed the reaction using glucose derived complexes,
which open the door of chiral synthesis.
3
3.2 Selective Oxidation of Sulfides to Sulphoxide
and Suphones
The structure of molybdenum complexes containing gly-
cosylamine derived Schiff bases has been already estab-
lished [10, 17], however their applications are yet to be
explored. Only one report is available on catalytic epoxi-
dation reactions [10] by cis-dioxo Mo (VI) complexes of
sugar derived chiral Schiff base ligands. To the best of our
knowledge there is no report on oxidation of sulfides to
sulphoxides or sulphones using similar saccharide derived
molybdenum complexes. Lack of such studies prompted us
to explore the sulfide oxidation reaction using sugar
derived molybdenum complexes. Along this line, firstly we
optimized the conditions for the oxidation of thioanisole to
methyl phenyl sulphoxide and methyl phenyl sulphone
using UHP and complex 1. Ethanolic solution of complex
(1), thioanisole and UHP in 0.05:1:1 ratio was stirred at
room temperature and progress of reaction was monitored
by TLC as well as HPLC. Building of peaks at 5.5 min in
the HPLC spectrum supported the formation of methyl
phenyl suphoxide at the cost of decrement of peak at
6.6 min, which corresponds to thioanisole (Fig. 2). The
pure methyl phenyl suphoxide was isolated by column
chromatography after 15 min of reaction time and it’s
4 Conclusion
We have synthesized a series of 4,6-O-ethylidene-b-D-
glucopyranosylamine derived ligands, their Mo (VI) com-
plexes and application of the latter in the oxidation of
thioanisole. This is the first report, where sugar derived Mo
(VI) complexes has been used in sulfide oxidation reaction.
Various aspects of the reaction like role of solvent, reaction
time, necessities of catalyst and UHP has been explored.
These preliminary success is getting extrapolated in the
chiral synthesis as our metal complex will act as chiral
catalyst due to its sugar origin.
Acknowledgments Ajay K. Sah is grateful to Department of Sci-
ence and Technology (DST) and University Grants Commission
(UGC) for the financial support. We also acknowledge UGC SAP
(Special Assistance Programme) and DST FIST (Fund for Improve-
ment of S&T Infrastructure in Higher Educational Institutions) to
provide the funding for procuring the equipments for analytical
studies.
1
purity was judged by HPLC (inset, Fig. 2) and H NMR
References
(refer supplementary data). Several control reactions were
performed to learn the effects of amounts of catalyst and
UHP on the product formation and the same is summarized
in Table 1. No oxidation was noticed in absence of either
catalyst or UHP and the same was also noticed by
Sheikhshoaie et al. [18]. Increase in the reaction time or
UHP concentration lead to over oxidation, forming the
sulphone. Selective sulphone formation was observed
when substrate to UHP concentration was taken in 1:5
ratio.
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After confirming the oxidation of thioanisole in ethanol,
several other solvent were tried and the results are sum-
marized in Table 2. The maximum conversion of thioani-
sole into methyl phenyl sulphoxide was observed in
ethanol, while minimum was in the toluene. Reaction
proceeds with lower rate and yield in less polar solvents,
while it is quicker and high yielding in alcohols.
The oxidation of thioanisole with rest of the complexes
(2–4) was performed under optimized condition and the
results are summarized in Table 3. In all the cases, reaction
time was limited to 15 min to avoid the sulphone forma-
tion. Our results are comparable with the report where
Sheikhshoaie et al. [18] have performed similar reaction
123