H. Kargar, M. Bazrafshan, M. Fallah-Mehrjardi et al.
Polyhedron 202 (2021) 115194
be regarded as an appropriate oxidant due to its inexpensive nat-
ure, safe storage, good solubility and icing on cake, it generates
water as a by-product [22–26]. But, the rate of oxidation reaction
by hydrogen peroxide is very low due to its weak oxidizing capa-
bility and hence it is needed to be activated by using some suitable
activating agents like sodium tungstate or hydrogen bromide
[27,28]. So, instead of using highly unsafe liquid H2O2, the solid
urea-hydrogen peroxide (UHP) in combination with oxovana-
dium(V) and dioxomolybdenum(VI) complexes is regarded as
safer, greener and suitable substitute due to its easy availability,
high stability and high hydrogen peroxide contents [29,30].
Hence, in culmination and continuation of our previous work
[31], we are hereby reporting a novel and effective procedure for
the selective oxidation of BzOH to aldehydes catalyzed by oxo-
vanadium(V) and dioxomolybdenum(VI) complexes using urea
hydrogen peroxide as oxidizing agent.
ing [VIVO(acac)2] (1 mmol, 0.265 g, acac = acetylacetonate) with
H2L (1 mmol, 0.320 g) in methanol (50 mL). The mixture was kept
under reflux for 3 h to obtain the solid product in the form of pre-
cipitates which were filtered off and then washed thoroughly with
equal amounts of water, methanol and diethyl ether, separately.
VOL(OMe): Yield 61%. Anal. Calc. for C14H11BrN3O4V: C, 40.41;
H, 2.66; N, 10.10, Found: C, 40.33; H, 2.69; N, 10.16%. FT-IR (KBr,
cmꢀ1); 1614 (
995 ( V=O); 499 (
m
C@N); 1460 (
m
C@N-N@C); 1286 (
mCAO); 1029 (mNAN);
m
m
V-O); 476 (m
V-N). 1H NMR (400 MHz, DMSO d6,
ppm): 3.88 [3H, s, (–OCH3)], 7.01 [1H, (H-C2), t, 3J = 8.9 Hz], 7.55–
7.59 [2H, (H-C12, H-C3), m], 7.88 [1H, (H-C5), d, 4J = 2.7 Hz], 8.32
[1H, (H-C13), dt, 3J = 8.0 Hz, 4J = 1.8 Hz], 8.77 [1H, s, (CH = N)],
8.98 [1H, (H-C11), br], 9.15 [1H, (H-C10), br]. 13C NMR (100 MHz,
DMSO d6, ppm): 66.3 (–OCH3), 120.6 (C4), 121.5 (C), 124.0 (C6),
124.9 (C12), 125.9 (C9), 133.0 (C5), 134.4 (C3), 135.5 (C13), 148.8
(C10), 152.5 (C11), 155.7 (C1), 158.1 (C7), 167.7 (C8).
2.2.3. Synthesis of MoO2L complex
2. Experimental
Equimolar amounts of H2L (1 mmol, 0.320 g) and [MoVIO2(-
acac)2] (1 mmol, 0.330 g) were suspended in 100 mL of methanol
in a round bottom flask equipped with a magnetic bar for steady
stirring to attain the uniformity. The mixture was kept under reflux
over a water bath for a period of 3 h until the solid product was
precipitated out. The MoO2L complex was then filtered off and
washed meticulously with equal amounts of water, methanol
and diethyl ether, separately, to remove impurities/by products if
any. The precipitates were dried in vacuo and finally crystallized
from CH3CN to get orange-colored crystals.
2.1. Material and methods
All the chemicals employed in the current work were 99.9%
pure and purchased from well renowned suppliers like Sigma-
Aldrich and Merck. Elemental analysis was carried out by Heraeus
CHN-O-FLASH EA 1112 instrument. 1H and 13C NMR spectra were
measured at ambient temperature by using BRUKER AVANCE
400 MHz spectrometer in the presence of tetramethylsilane
(TMS) as an internal standard. Coupling constant (J) and chemical
shift (d) values were reported in Hz and in ppm, respectively. Four-
ier transform infrared spectra of the synthesized compounds were
taken, by making KBr pellets, with the help of IR Prestige-21
(Shimadzu).
MoO2L: Yield 67%. Anal. Calc. for C13H8BrMoN3O4: C, 35.00; H,
1.81; N, 9.42, Found: C, 35.12; H, 1.87; N, 9.33%. FT-IR (KBr,
cmꢀ1); 1616 (
935 (
m
C@N); 1462 (
m
C@N-N@C); 1265 (
m
m
CAO); 1018 ( NAN);
Mo-O); 480 (mMo-N).
m
m
O=Mo=O) asym; 914 ( O=Mo=O) sym; 569 (
m
1H NMR (400 MHz, DMSO d6, ppm): 6.95 [1H, (H-C2), t,
3J = 8.8 Hz], 7.57 [1H, (H-C12), dd, 3J = 7.3 Hz, 3J = 4.7 Hz], 7.68
[1H, (H-C3), dd, 3J = 8.8 Hz, 4J = 2.6 Hz], 8.00 [1H, (H-C5), d,
4J = 2.6 Hz], 8.32 [1H, (H-C13), d, 3J = 8.0 Hz], 8.78 [1H (H-C11),
br], 8.98 [1H, s, (CH = N)], 9.16 [1H (H-C10), br]. 13C NMR
(100 MHz, DMSO d6, ppm): 112.4 (C2), 120.9 (C6), 122.1 (C12),
124.0 (C4), 125.9 (C9), 135.5 (C5), 136.0 (C3), 137.2 (C13), 148.8
(C10), 152.5 (C11), 155.6 (C1), 158.5 (C7), 167.7 (C8).
2.2. Synthesis
2.2.1. Synthesis of ONO-tridentate Schiff base ligand (H2L)
Nicotinic hydrazide (1.37 g, 10 mmol) and 5-bromosalicylalde-
hyde (2.01 g, 10 mmol) were dissolved separately in approximately
25 mL of hot methanol. After complete dissolution, both solutions
were mixed dropwise with continuous stirring. The resulting mix-
ture was refluxed for 3 h and the completion of reaction was
ensured by ongoing monitoring with the help of TLC. On allowing
the reaction mixture to attain the room temperature, the product
was settled down leaving behind the impurities in the solvent.
Finally, the desired product was collected by filtration, aided by
suction apparatus and washed thrice with cold methanol to
remove impurities if any. The prepared H2L was finally crystalized
from absolute ethanol to get the single crystal suitable for SC-XRD
analysis.
2.3. X-ray crystallographic data collection H2L ligand and MoO2L
complex
Single crystal X-ray studies of H2L and MoO2L were carried out
on a STOE IPDS-II diffractometer by using Mo-K
monochromated by graphite. The data was collected at 298(2) K
in a series of -scans in 1° oscillations and integrated by using
a radiations
x
the Stöe X-AREA [32] software package. A numerical absorption
correction was applied using the X-RED [33] and X-SHAPE [34]
software for the prepared compounds. The data was corrected for
Lorentz and Polarizing effects. The structures were solved by direct
methods using SIR2004 [35]. The non-hydrogen atoms were
refined anisotropically by the full-matrix least-squares method
on F2 using SHELXL [36]. The collected crystallographic data of
the ligand and its Mo complex are listed in Table S1.
H2L: Yield 75%. Anal. Calc. for C13H10BrN3O2: C, 48.77; H, 3.15;
N, 13.13, Found: C, 48.84; H, 3.19; N, 13.21%. FT-IR (KBr, cmꢀ1);
3269 (
m
N–H); 1678 (
mC=O); 1610 (mC=N); 1591, 1477 (mC=c); 1188
(m
C-O); 1026 (m
N-N). 1H NMR (400 MHz DMSO d6, ppm): 6.91 [1H,
(H-C2), d, 3J = 8.8 Hz], 7.44 [1H, (H-C3), dd, 3J = 8.8 Hz, 4J = 2.4 Hz],
7.58 [1H, (H-C12), dd, 3J = 7.8 Hz, 3J = 4.9 Hz], 7.82 [1H, (H-C5), d,
4J = 2.4 Hz], 8.28 [1H, (H-C13), dd, 3J = 6.5 Hz, 4J = 1.7 Hz], 8.63
[1H, s, (CH = N)], 8.78 [1H, (H-C11), d,3J = 3.9 Hz], 9.09 [1H, (H-
C10) br], 11.18 [1H, s, (–NH)], 12.32 [1H, s, (–OH)]. 13C NMR
(100 MHz, DMSO d6, ppm): 110.5 (C2), 118.7 (C6), 121.3 (C12),
123.6 (C4), 128.6 (C9), 130.2 (C5), 133.7 (C3), 135.5 (C13), 145.9
(C7), 148.6 (C10), 152.4 (C11), 156.4 (C1), 161.6 (C8).
2.4. Computational details
Density functional theory (DFT) calculations were performed
with the Gaussian 09 package [37] at B3LYP level of theory [38]
by using Def2-TZVP basis set [39]. The solution phase was mod-
eled by using IEFPCM with the consideration of solvent (CH3CN)
[40]. Geometry optimizations were tested by frequency analysis
to ensure that they are at the local minima on the molecular
potential energy surface (PES). The results showed that there is
2.2.2. Synthesis of VOL(OMe) complex
The new VOL(OMe) complex, where L = (E)-N’-(5-bromo-2-
hydroxybenzylidene)nicotinohydrazide, was synthesized by treat-
2