H. Kargar et al.
Inorganica Chimica Acta 523 (2021) 120414
greater quantity of these oxidizing agents but also associated with the
production of toxic side products [25,26]. In addition to this, the
oxidation reactions require harsh reaction conditions like, elevated
pressure of oxygen, high temperature and the use of unavoidable toxic
solvents which are not only corrosive but also very dangerous from an
environmental point of view [27]. In order to overcome above
mentioned issues, a lot of manipulations in reaction conditions were
carried out to control the rate of reaction and selectivity, accompanying
the amount and nature of by-products produced [28-30]. These trans-
formations include changes in the concentration of reactants, pressure,
temperature, alternative environmentally friendly solvents, and to
design and develop novel catalysts [31-34].
(
(
νN–H); 1631 (νC=O); 1600 (νC=N); 1552, 1338 (νN=O); 1294 (νC-O); 1076
νN-N). 1H NMR (400 MHz, DMSO‑d6, ppm): 7.08 [1H, (H-C2), d, 3J =
9.0 Hz], 7.57 [1H, (H-C12), dd, 3J = 7.6 Hz, 3J = 4.9 Hz ], 8.14 [1H, (H-
C3), dd, 3J = 9.0 Hz, 4J = 2.7 Hz], 8.28 [1H, (H-C13), d, 3J = 7.6 Hz],
8.57 [1H, (H-C5), d, 4J = 2.7 Hz], 8.71 [1H, s, (–CH = N)], 8.77 [1H, (H-
C11), br], 9.09 [1H, (H-C10), br], 12.09 [1H, (–NH), s], 12.35 [1H,
(–OH), s]. 13C NMR (100 MHz, DMSO‑d6, ppm): 116.68 (C2), 117.02
(C6), 119.87 (C12), 123.56 (C3), 126.62 (C5), 128.47 (C9), 135.45
(C13), 139.87 (C4), 144.63 (C7), 148.64 (C10), 152.47 (C11), 161.62
(C8), 162.50 (C1).
2.2.2. Synthesis of [VVO(L)(OCH3)(CH3OH] complex
Consequently, catalytic oxidation schemes based on transition
metals like copper, nickel, cobalt, manganese, iron, palladium, chro-
mium and gold [35-41], along with molecular oxygen [42,43],
hydrogen peroxide [44] and tert-butyl hydroperoxide (TBHP) [45,46],
as oxidizing agents, were investigated.
The new [VvO(L)(OCH3)(CH3OH)] complex, VOL, where L = (E)-N’-
(5-nitro-2-hydroxybenzylidene)nicotinohydrazide, was synthesized by
treating VO(acac)2 (1 mmol, 0.265 g, acac = acetylacetonate) with H2L
(1 mmol, 0.286 g) in methanol (50 mL). The mixture was kept under
reflux for 3 h to obtain the resultant product in the form of precipitates
which were filtered off and then washed thoroughly with equal amounts
of water, methanol and diethyl ether. The precipitates were dried in
vacuo and then crystallized from CH3OH to get green colored crystals
suitable for single crystal analysis.
In addition to above listed transition metal ions having high catalytic
potential with H2O2, the metals with high oxidation states like oxova-
nadium(V) and dioxomolybdenum(VI), have shown better potential for
catalytic oxidation of benzyl alcohol owing to their greater attribution
with peroxide to make catalytically active intermediates [47-54]. These
metal peroxy intermediates can easily shift the oxygen atom to the
substrate to oxidize it [55]. Keeping in mind the greater affinity of VO
(V) and MoO2(VI) complexes in the catalytic oxidation reactions, it can
be predicted that oxidation of benzylic alcohols to aldehydes or ketones
in the presence of urea hydrogen peroxide under lenient reaction con-
ditions can carried out with the help of oxovanadium(V) and dioxo-
molybdenum(VI) complexes [56].
[VvO(L)(OCH3)(CH3OH)]: Yield 61%. Anal. Calc. for C15H15N4O7V:
C, 43.49; H, 3.65; N, 13.53, Found: C, 43.37; H, 3.59; N, 13.57%. FT-IR
(KBr, cmꢀ 1); 3410 (νO-H) (coordinated methanol); 1604 (νC=N); 1462
(
(
νC=N-N=C); 1546, 1336 (νN=O); 1313 (νC-O); 1028 (νN-N); 918 (νV=O); 580
νV-O); 457 (νV-N). 1H NMR (400 MHz, DMSO‑d6, ppm): 3.17 [3H,
(–OCH3 Coord.), s], 3.37 [3H, (MeOH Coord.), d, 3J = 6.9 Hz], 4.07 [1H,
(MeOH Coord.), q, 3J = 6.9 Hz], 7.11 [1H, (H-C2), d, 3J = 8.8 Hz],
7.65–7.69 [2H, (H-C12, H-C3), m], 7.98 [1H, (H-C5), d, 4J = 2.7 Hz],
8.42 [1H, (H-C13), dt, 3J = 8.0 Hz, 4J = 1.8 Hz], 8.87 [1H, (H-C11), br],
9.09 [1H, (CH = N), s], 9.25 [1H, (H-C10), br]. 13C NMR (100 MHz,
DMSO‑d6, ppm): 64.4 (C (MeOH Coord.)), 66.3 (C (–OCH3 Coord.)),
120.7 (C2), 121.5 (C6), 124.1 (C12), 124.9 (C9), 126.1 (C3), 133.1 (C5),
134.5 (C13), 135.7 (C4), 149.0 (C10), 152.7 (C7), 155.9 (C11), 158.2
(C1), 167.8 (C8).
In continuation to our previous researches and work on Schiff base
complexes for oxidation of organic compounds [57-60], we are hereby
describing the synthesis, spectroscopic characterization, crystal struc-
ture, computational studies and catalytic potentials of two novel oxo-
vanadium(V) and dioxomolybdenum(VI) complexes derived from
nicotinic hydrazides.
2. Experimental
2.2.3. Synthesis of [MoVIO2(L)(CH3CH2OH)] complex
For the synthesis of new [MoVIO2(L)(CH3CH2OH)] complex, MoO2L,
equimolar amounts of H2L (1 mmol, 0.286 g) and MoO2(acac)2 (1 mmol,
0.330 g) were suspended in 100 mL of methanol in a round bottom flask
containing a magnetic bar for steady stirring. The mixture was kept
under reflux for 3 h to obtain the product in the form of precipitates
which were filtered off and then washed thoroughly with equal amounts
of water, methanol and diethyl ether, separately. The precipitates were
dried in vacuo and then crystallized from CH3CH2OH to get orange-
colored crystals.
2.1. Materials 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 tem-
perature by using BRUKER AVANCE 400 MHz spectrometer by using
tetramethylsilane (TMS) as an internal standard. Coupling constant (J)
and chemical shift (δ) values were reported in Hz and in ppm, respec-
tively. Fourier transform infrared spectra of the synthesized compounds
were recorded by making their KBr pellets with the help of IRPrestige-21
(Shimadzu) spectrophotometer.
[MoVO2(L)(CH3CH2OH)]:
Yield
72%.
Anal.
Calc.
for
C
15H14MoN4O7: C, 39.32; H, 3.08; N, 12.23, Found: C, 39.23; H, 3.13; N,
12.31%. FT-IR (KBr, cmꢀ 1); 3448 (νO-H) (coordinated ethanol); 1612
(
νC=N); 1467 (νC=N-N=C); 1544, 1334 (νN=O); 1263 (νC-O); 1033 (νN-N);
933 (νO=Mo=O) asym; 916 (νO=Mo=O) sym; 597 (νMo-O); 476 (νMo-N). 1
H
NMR (400 MHz, DMSO‑d6, ppm): 1.05 [3H, (–CH3 EtOH Coord.), t, 3J =
6.9 Hz], 3.45 [2H, (–OCH2 EtOH Coord.), m], 4.33 [1H, (–OH EtOH
Coord.), br], 7.17 [1H, (H-C2), t, 3J = 9.1 Hz], 7.58 [1H, (H-C12), dd, 3J
= 7.5 Hz, 3J = 4.7 Hz], 8.33–8.36 [2H, (H-C3, H-C13), m], 8.81 [1H (H-
C11), br], 8.82 [1H, (H-C5), d, 4J = 2.9 Hz], 9.18 [1H (H-C10), br], 9.19
[1H, (CH = N), s]. 13C NMR (100 MHz, DMSO‑d6, ppm): 19.0 (C (–CH3
EtOH Coord.)), 56.73 (C (–OCH2 EtOH Coord.)), 120.13 (C2), 120.20
(C6), 124.04 (C12), 125.68 (C9), 129.56 (C3), 130.51 (C5), 135.60
(C13), 140.93 (C4), 148.87 (C10), 152.69 (C11), 155.98 (C7), 163.93
(C1), 167.85 (C8).
2.2. Synthesis
2.2.1. Synthesis of ONO-tridentate Schiff base ligand (H2L)
Nicotinic hydrazide (1.37 g, 10 mmol) and 5-nitrosalicylaldehyde
(1.67 g, 10 mmol) were dissolved separately in an approximately 25
mL of hot methanol. After complete dissolution both solutions were
mixed dropwise with continuous stirring. The resulting mixture was
refluxed for round about 3 h until 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.
H2L: Yield 75%. Anal. Calc. for C13H10N4O4: C, 54.55; H, 3.52; N,
19.57, Found: C, 54.44; H, 3.55; N, 19.64%. FT-IR (KBr, cmꢀ 1); 3302
2.3. X-ray crystallographic analysis, data collection and structure
determination of complexes
Single crystal X-ray studies of oxovanadium and dioxomolybdenum
2