H. Kargar et al.
Polyhedron 208 (2021) 115428
solvents. Many synthetic routes have been developed until now to
accomplish the anticipated transformations by using the appropriate
quantity of various oxidants [13]. The conventional methods adapted
for the oxidation process involve the use of inorganic oxidants like
chromates, chlorates permanganates, and oxides of various transition
metals etc. [14,15]. Hence, a mixture consisting of organic substrates,
solvents (usually chlorinated hydrocarbons), oxidants and products is
very familiar with creating unavoidable explosions in several cases, as
these protocols are performed under severe conditions of temperature
and pressure [16].
product at a particular Rf value. After complete satisfaction with the
accomplishment of the reaction, the contents were allowed to cool.
Cooling of the reaction contents decreases the solubility and the product
descends in the form of precipitates. The precipitates were then sorted
out by the process of filtration under reduced pressure with the help of a
rotary evaporator and cleansed in triplicate with cold methanol to purify
them.
H
2
L: Yield 67%. Anal. Calc. for C15
15 3 4
H N O : C, 59.80; H, 5.02; N,
ꢀ 1
13.95, Found: C, 59.91; H, 5.06; N, 14.07%. FT-IR (KBr, cm ); 3470
–
–
1
(
ν
N
–
H
); 1645 (
ν
C
O
); 1605 (
ν
C
–
N
); 1159 (
ν
C
–
O
); 1078 (
ν
N
–
N
). H NMR
NH )],
6.03 [1H, (H-C12), d, J = 2.0 Hz], 6.14 [1H, (H-C14), dd, J = 8.8 Hz,
–
Keeping in mind these restrictions, scientists are always excited
about the development of environmentally friendly and harmless cata-
lytic procedures for the oxidation of alcohols using some transition
(400 MHz, DMSO‑d
6
, ppm): 3.81 [3H, s, (H-C7)], 5.99 [2H, s, (
–
2
4
3
4
3
3
J = 2.0 Hz], 6.86 [1H, (H-C4), t, J = 8.0 Hz], 7.02 [1H, (H-C3), dd, J
4
3
4
metal complexes together with O
2
, H
2
O
2
, tert-BuOOH and, most pref-
= 8.0 Hz, J = 1.2 Hz], 7.12 [1H, (H-C5), dd, J = 8.0 Hz, J = 1.2 Hz],
3
–
–
N)], 11.07 [1H,
erably, solvent free routes [17]. Solvent free organic reactions have
attained greater attraction due to several advantages, like high catalytic
efficiency, selectivity, easy way of separation, non-tedious reaction
conditions, simplicity and easy to work out [18–21].
7.63 [1H, (H-C15), d, J = 8.8 Hz], 8.60 [1H, s, (
–
CH
1
3
s, (
–
NH)], 11.76 [1H, s, (
–
OH)], 12.35 [1H, s, (
–
OH)]. C NMR (100
6
MHz, DMSO‑d , ppm): 55.8 (C7) 99.3 (C12), 101.3 (C10), 105.9 (C14),
113.6 (C3), 118.9 (C6), 119.0 (C4), 120.8 (C5), 128.9 (C15), 147.0 (C2),
147.4 (C1), 147.9 (C8), 154.8 (C13), 162.8 (C11), 165.8 (C9).
The dioxidomolybdenum(VI) complexes have been successful in
capturing the eyes of scholars due to their flexible approach towards
reactivity, selectivity, and catalytic significance [22,23]. These dioxi-
domolybdenum(VI) complexes with aroylhydrazones have been studied
extensively for various homogeneous and heterogeneous catalytic re-
actions, including selective oxidation of benzyl alcohols, epoxidation
and sulfoxidation [24,25]. In these types of dioxo complexes, the ligand
is usually doubly deprotonated and behaves in a tridentate fashion, and
the sixth coordination site is usually occupied by the solvent molecule to
form mononuclear complexes [23]. The attached solvent molecule is
labile and facilitates the attachment of substrate molecules to complete
the catalytic cycle [7].
VI
2.2.2. Preparation of [Mo O
2
L(H
2
O)] complex
VI
The dioxidomolybdenum complex [Mo
by suspending equimolar amounts of ligand H
along with MoO (acac) (0.5 mmol, 0.165 g) in 50 mL of MeOH in a two
O
2
L(H
2
O)] was prepared
2
L (0.5 mmol, 0.150 g)
2
2
necked flask with magnetic stirring over a hot plate. The stuffing of the
suspension was refluxed for roughly three hours. After that, it was
concentrated with the help of a rotary evaporator by creating an envi-
ronment of reduced pressure to facilitate the evaporation of excess sol-
vent. At the end, the flask was ice-cooled to procure the orange-coloured
precipitates, which were then filtered and cleansed meticulously with
diethyl ether and water, and then dehydrated in a desiccator under an
inert atmosphere. A saturated solution of the prepared complex was
made by dissolving it into a minimum volume of dimethyl formamide
Our research group has been engaged in the synthesis, character-
ization, catalytic potential and DFT studies of many of the Schiff base
transition metal complexes in the last few years [26–30]. The present
work is about the exploration of catalytic potential of the prepared
dioxidomolybdenum(VI) complex for the conversion of benzyl alcohols
to aryl aldehydes via tert-butyl hydroperoxide without the involvement
of solvents.
(DMF) to grow the crystals of the subject complex (MoO
2
L(H
2
O).2DMF)
suitable for single crystal analysis.
[MoO
2
L(H
2
O)]: Yield 73%. Anal. Calc. for C15
H15MoN
3 7
O : C,
40.46; H, 3.40; N, 9.44, Found: C, 40.25; H, 3.34; N, 9.51%. FT-IR (KBr,
ꢀ
1
–
– –
C N-N C
); 1456 ( ); 1253 (
cm ); 1597 (
ν
C
N
ν
ν
Mo
C
–
–
O
); 1055 (
); 451 (
6
, ppm): 3.81 [3H, s, (H-C7)], 6.04 [2H, s,
ν
N N
–
); 933
ν ). H
–
Mo N
–
– –
– –
–
–
1
2
. Experimental
(
ν
O
Mo
O
) asym; 912 (
ν
O
– –
Mo O
) sym; 599 (
ν
O
–
–
NMR (400 MHz, DMSO‑d
NH
)], 6.08 [1H, (H-C12), d, J = 2.0 Hz], 6.19 [1H, (H-C14), dd, J
8.8 Hz, J = 2.0 Hz], 7.02 [1H, (H-C4), t, J = 8.0 Hz], 7.21 [1H, (H-
4
3
2
.1. Materials and methods
(
–
=
2
4
3
3
3
Each and every chemical and solvent utilized in the current work is
C3), d, J = 8.0 Hz], 7.23 [1H, (H-C5), d, J = 8.0 Hz], 7.44 [1H, (H-
3
–
–
–
CH N)], 11.32 [1H, s, (
of reagent grade and procured from Acros Organics, Merck, and Sigma-
Aldrich. The elemental composition (CHN) was determined by using
C15), d, J = 8.8 Hz], 8.87 [1H, s, (
–
OH)].
1
3
6
C NMR (100 MHz, DMSO‑d , ppm): 55.8 (C7), 99.2 (C12), 100.7
1
Heraeus CHN-O-FLASH EA 1112 equipment. The multinuclear ( H &
(C10), 106.8 (C14), 116.4 (C3), 120.6 (C6), 121.6 (C4), 124.7 (C5),
130.4 (C15), 148.3 (C2), 148.6 (C1), 152.8 (C8), 154.8 (C13), 160.6
(C11), 169.7 (C9).
1
3
C) NMR data was collected with the assistance of a BRUKER AVANCE
4
00 MHz spectrometer. The chemical shift values (δ) are assigned by
comparing them with tetramethylsilane (TMS), an internal reference,
whose value is arbitrarily fixed as zero ppm. The different functional
groups present in the synthesized compounds were confirmed by
running their infrared spectra after preparing potassium bromide pellets
with the support of the IRPrestige-21 Spectrophotometer.
2.3. X-ray crystallographic data collection and structure determination of
complex
The diffraction data of the dioxidomolybdenum(VI) complex
(
2 2
MoO L(H O).2DMF) was collected on the Bruker Kappa APEX-II CCD
2
2
.2. Preparation
refractometer by employing molybdenum as a target material for the
production of X-rays. A graphite-based monochromator is used for the
.2.1. Preparation of (E)-4-amino-2-hydroxy-N’-(2-hydroxy-3-
generation of Mo-K radiation. The data was collected with the help of
α
methoxybenzylidene)benzohydrazide (H
L was prepared by dissolving 4-amino-2-hydroxybenzohydrazide
31] (10 mmol, 1.67 g) and 3-methoxysalicylaldehyde (10 mmol,
.52 g) separately in 50 mL of hot MeOH. The reactants were then
2
L)
APEX-II software [32], while SADABS [33] software was employed for
performing absorption corrections. The obtained raw data is resolved
with the help of SHELXS-97 [34] software by using direct methods,
while refinements of the data are executed by utilizing the full-matrix
least-squares method on F2 in SHELXL [35] software. Anisotropic
displacement parameters were applied for the refinement of all atoms
except hydrogen atoms. The placement of H-atoms at an ideal position
was determined with the help of relative isotropic displacement pa-
rameters. For the graphical illustrations of the collected X-ray data of the
H
2
[
1
mixed-up drop by drop and the resultant mixture was refluxed for
approximately three hours until the reactants were completely con-
verted into products as monitored by thin layer chromatography. For
this, aliquots from the mixture were taken periodically to look for the
disappearance of spots of reactants and the emergence of new spot of
2