Green synthesis of a vanadium(V) Schiff base complex by grinding method: study on its catalytic and anti-bacterial activity

Article abstract of DOI:10.1080/00958972.2021.1942861

A facile protocol was developed for the green synthesis of a Schiff base and its vanadium(V) complex. The compounds were derived from salicylaldehyde, diethylenetriamine and vanadyl sulfate by applying grinding method to overcome environmental issues with solvents. The reactions took place rapidly and products were isolated pure. The compounds were characterized with elemental, EDX, mass spectrometry, magnetic susceptibility, molar conductance, UV-vis, FTIR and NMR analysis. The synthesized vanadium(V) complex was an efficient and selective catalyst towards oxidation of organic sulfides and alcohols. The oxidation reactions were conducted under microwave irradiation as well as conventional heating. A reaction mechanism has been proposed for the catalytic oxidation reactions. The mechanism was further established by using GAUSSIAN16 suite. The complex also showed good anti-bacterial activity against Gram-positive and Gram-negative bacteria.

Full text of DOI:10.1080/00958972.2021.1942861

Journal of Coordination Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Green synthesis of a vanadium(V) Schiff base
complex by grinding method: study on its catalytic
and anti-bacterial activity
Jeena Jyoti Boruah, Zankhana S. Bhatt, Chirag R. Nathani, Vaishali J.
Bambhaniya, Ankur Kanti Guha & Siva Prased Das
To cite this article: Jeena Jyoti Boruah, Zankhana S. Bhatt, Chirag R. Nathani, Vaishali J.
Bambhaniya, Ankur Kanti Guha & Siva Prased Das (2021) Green synthesis of a vanadium(V)
Schiff base complex by grinding method: study on its catalytic and anti-bacterial activity, Journal of
Coordination Chemistry, 74:12, 2055-2068, DOI: 10.1080/00958972.2021.1942861
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JOURNAL OF COORDINATION CHEMISTRY
2021, VOL. 74, NO. 12, 2055–2068
https://doi.org/10.1080/00958972.2021.1942861
Green synthesis of a vanadium(V) Schiff base complex
by grinding method: study on its catalytic and
anti-bacterial activity
Jeena Jyoti Boruaha,b†, Zankhana S. Bhatta, Chirag R. Nathania, Vaishali J.
Bambhaniyaa, Ankur Kanti Guhac and Siva Prased Dasa†
aDepartment of Chemistry, School of Science, RK University, Kasturbadham, Rajkot, Gujarat, India;
c
bDepartment of Chemistry, Moridhal College, Dhemaji, Assam, India; Advanced Computational
Chemistry Centre, Department of Chemistry, Cotton University, Panbazar, Guwahati, Assam, India
ABSTRACT
ARTICLE HISTORY
Received 30 November 2020
Accepted 28 May 2021
A facile protocol was developed for the green synthesis of a
Schiff base and its vanadium(V) complex. The compounds were
derived from salicylaldehyde, diethylenetriamine and vanadyl sul-
fate by applying grinding method to overcome environmental
issues with solvents. The reactions took place rapidly and prod-
ucts were isolated pure. The compounds were characterized with
elemental, EDX, mass spectrometry, magnetic susceptibility, molar
conductance, UV-vis, FTIR and NMR analysis. The synthesized
vanadium(V) complex was an efficient and selective catalyst
towards oxidation of organic sulfides and alcohols. The oxidation
reactions were conducted under microwave irradiation as well as
conventional heating. A reaction mechanism has been proposed
for the catalytic oxidation reactions. The mechanism was further
established by using GAUSSIAN16 suite. The complex also showed
good anti-bacterial activity against Gram-positive and Gram-nega-
tive bacteria.
KEYWORDS
Schiff base; vanadium(V)
complex; grinding method;
green synthesis; anti-
bacterial activity;
catalytic activity
CONTACT J. J. Boruah
Siva.das@Rku.ac.in
Jeena.jyoti@Gmail.com or Jeena.jyoti@Rku.ac.in; S. P. Das
Department of Chemistry, School of Science, RK University, Bhavnagar Highway,
Siva.spd@Gmail.com or
Kasturbadham, Rajkot-360020, Gujarat, India.
†Authors contributed equally to this work.
Supplemental data for this article is available online at https://doi.org/10.1080/00958972.2021.1942861.
ß 2021 Informa UK Limited, trading as Taylor & Francis Group

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J. J. BORUAH ET AL.
1. Introduction
With the tightening of environmental legislation, there has been a demand of environ-
mentally benign chemical processes. One approach is utilization of mechanochemical
or grinding reactions. IUPAC defined this class of reactions as “a chemical reaction that
is induced by the direct absorption of mechanical energy” [1]. Usually, in mechano-
chemistry, the mechanical energy produced by the shock or friction between two dif-
ferent surfaces is utilized in chemical synthesis [2]. Typically, a mortar and pestle is
used to generate enough mechanical energy to achieve the desired synthesis [2, 3].
However, a ball mill is preferred over mortar and pestle for reactions that require lon-
ger time. Solvent-less reaction conditions, minimum waste, faster reactions, wider sub-
strate scope, cheaper, etc. are the advantages of this method [3]. Thus, the method
complies many principles of “Green Chemistry” [4]. The liquid-assisted grinding (LAG)
reaction is another version of mechanochemical or grinding reaction where little solv-
ent is introduced into the reaction mixture to drive the reaction [5].
Schiff bases, a fascinating class of compounds, are widely used in catalysis, food
industry, analytical chemistry, dye industry, agrochemical, fungicidal, etc. [6–11]. They
are formed from condensation of carbonyl compounds and amines. The imine group
of Schiff base coordinates with the metal via N or double bond. Metal complexes have
better biological activity than the free Schiff base [12]. Thus, syntheses of Schiff bases
and their metal complexes by different techniques are important, but many of these
methods have severe harsh refluxing conditions, use of excess organic solvent, pro-
longed reaction time, lower yield, etc., not acceptable from Green Chemistry aspects
[4]. Development of more efficient and environmentally benign preparation methods
for Schiff bases and their metal complexes are needed. Use of mechanochemical or
grinding reactions for the synthesis of Schiff base and their metal complexes will help
to comply with many postulates of Green Chemistry.
Vanadium has electronic configuration [Argon]4s23d3 and thus it forms compounds
with oxidation states from þ5 (d0) to ꢀ3 (d8). It is the 21st most abundant element in
the earth’s crust [13] and 2nd most abundant transition metal in sea water [14]. It has
drawn attention due to its rich chemistry in catalysis and variety of biological func-
tions. In catalysis, vanadium complexes are oxidizing or oxo-transfer agents in a variety
of organic transformations such as oxidations of alkanes, alkenes, arenes, alcohols,
aldehydes, ketones, sulfur species, etc. [15]. Vanadium is ubiquitous in nature and
influences a wide variety of biological activities [16, 17]. Vanadium complexes have
anti-diabetic [18], insulin mimetic [18–20], anti-cancer [21] and enzyme inhibitory activ-
ities [21–23]. Beside these, there are different types of enzymes available naturally
such as nitrogenases, haloperoxidases [24, 25], phosphate-metabolizing enzymes [26]
and phosphomutases [27], whose activities depend on vanadium. Vanadium com-
plexes also have interesting antibacterial [28], antifungal [29, 30] and antiviral activities
[31]. Thus, synthesis of active vanadium complexes continues by researchers.
Herein, we report the synthesis of a Schiff base (H2L) and its vanadium(V) complex
(Na[VO2L]) by grinding method from easily available starting materials. The Schiff
base was derived from condensation of salicylaldehyde and diethylenetriamine. No
acid is required in the synthesis. The compounds were studied for their antibacterial
activities. We also report the catalytic activity of Na[VO2L] in selective oxidation of

JOURNAL OF COORDINATION CHEMISTRY
2057
organic sulfides and alcohols under microwave conditions as well as conventional
heating methods.
2. Experimental
The elemental analysis of the compounds were analyzed by using a Perkin-Elmer 2400
series II CHN analyzer (for C, H and N content). The vanadium content was determined
by volumetric methods [32]. The C, N, O, Na and V contents in the compounds were
also determined by EDX analysis. The infrared spectra of the compounds were
recorded from 4000 to 400 cmꢀ1 using KBr discs with a Nicolet model 410 FTIR spec-
trophotometer. UV-Vis spectra of the compounds were recorded in DMSO using a
1
Shimadzu U-2550 UV–Vis spectrophotometer. The H NMR spectra were recorded at
room temperature in CDCl3, MeOD-d4 or DMSO-d6 using a Bruker AVANCE 400 MHz
spectrometer. The solvent resonance peak was used as an internal standard. Magnetic
susceptibility of Na[VO2L] was measured by the Gouy method using Hg[Co(NCS)] as
the calibrant. The molar conductance was determined on a Labtronics Digital
Conductivity meter (Model-LT16). All chemicals were of reagent grade, purchased from
commercial sources and used without purification.
2.1. Synthesis of H2L and Na[VO2L]
The Schiff base and its vanadium(V) complex were synthesized in two separate steps
as described below.
2.1.1. Step-I: Synthesis of Schiff base (H2L)
H2L was synthesized by the grinding method. Salicylaldehyde (0.24 g, 2.00 mmol) and
diethylenetriamine (0.10 g, 1.00 mmol) were taken in a mortar and ground with a pes-
tle at room temperature. The progress of the reaction was monitored by TLC. The
reaction mixture became yellow instantly. As indicated by TLC, the reaction was com-
pleted in 5 min of grinding. A yellow sticky product was obtained. Isolated yield 99%.
2.1.2. Step-II: Synthesis of vanadium(V) Schiff base complex (Na[VO2L])
Na[VO2L] was synthesized by the LAG method. H2L (0.25 g, 0.80 mmol) and 0.25 mL of
methanol/DMSO (1:1 mixture) were mixed in a mortar and pestle to make a pasty
mass. Subsequently, VOSO4ꢁ5H2O (0.20 g, 0.80 mmol) and NaOH (0.064 g, 1.60 mmol)
were added and ground at room temperature. The progress of the reaction was moni-
tor by TLC. After 30 min of grinding, the reaction mixture was allowed to stand in the
mortar for an additional 15 min with occasional grinding. The initial green color of the
reaction mixture became yellowish brown. The product thus obtained was washed
with hexane (3 ꢂ 5 mL) and dried under vacuum to get a powdery product. Isolated
yield 98%. Attempts were made to isolate single crystals suitable for X-ray structural
analysis, but were not successful.

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J. J. BORUAH ET AL.
2.2. Anti-bacterial activity of H2L and Na[VO2L]
The anti-bacterial activity of H2L and Na[VO2L] was performed in-vitro by the zone
inhibition method using agar diffusion. In the typical process, effect of the synthesized
compounds were studied against Gram-positive (S. aureus, B. subtilis) and Gram-nega-
tive (E. coli, P. aeruginosa) bacterial strains. According to this method, the compounds
were loaded on an agar plate with the bacterial strain and allowed to incubate for
24 h at 37 ꢃC. After that the diameter of inhibition by the compounds were measured
in millimeters.
2.3. General procedure for catalytic oxidation by Na[VO2L] under microwave
irradiation
All reactions were conducted at 100 W microwave power with 1.00 mmol substrate. In
the representative procedure, for selective oxidation of benzyl alcohol to benzalde-
hyde, benzyl alcohol (0.11 g, 1.00 mmol), 30% H2O2 (0.13 mL, 1.10 mmol) and Na[VO2L]
(0.83 mg, 0.002 mmol) were taken in a 50 mL conical flask covered with an inverted
funnel. The molar ratio of substrate:H2O2:Na[VO2L] is maintained at 500:550:1. The
progress of the reaction was monitored by TLC. After completion of the reaction, the
products were isolated from the reaction mixture with ethyl acetate (thrice). The com-
bined extracts were dried over anhydrous Na2SO4 and excess solvents were removed
under reduced pressure. Identification of products was from comparing Rf values with
authentic samples. The compounds were also characterized by NMR analysis.
Similar reaction procedure is used for selective oxidation of benzyl alcohol to ben-
zoic acid, methyl phenyl sulfide to methyl phenyl sulfoxide and methyl phenyl sulfide
to methyl phenyl sulfone by keeping the molar ratio of substrate:H2O2:Na[VO2L] at
500:1500:1, 500:525:1 and 500:1250:1, respectively.
2.4. General procedure for catalytic oxidation by Na[VO2L] under heating
All reactions were conducted in a 25 mL round bottom flask under magnetic stirring
at 70 ꢃC with 1.0 mmol substrate. The molar ratios of substrate:H2O2:Na[VO2L] were
kept as mentioned above. The progress of the reaction, isolation and characterization
of product were done as mentioned above.
2.5. Computational details
All the structures were fully optimized without any symmetry constraints at M06-2X/6-
ꢄꢄ
311þþG level of theory [33]. Harmonic vibrational frequency calculations were car-
ried out at the same level of theory to understand the nature of the stationary points.
All the structures were found to be in their local minima with all real values of the
Hessian matrix. The energies are zero point and thermal corrected. All calculations
were performed using GAUSSIAN16 suite of programs [34].

JOURNAL OF COORDINATION CHEMISTRY
2059
Scheme 1. Synthesis of H2L and Na[VO2L].
3. Results and discussion
3.1. Synthesis
We have synthesized H2L by mechanochemical or grinding method as shown in
Scheme 1 which was not reported earlier [35–39]. The advantages of grinding method
are outlined in the Introduction (vide supra). The process is simple, solventless,
required no additives and environmentally benign. Specially, the method is acid free.
The synthesis was achieved by grinding the mixture of salicylaldehyde and diethylene-
triamine at molar ratio of 2:1 in a mortar and pestle at room temperature for 5 min
with isolated yield of 99%. The product is pure as confirmed by different analytical
techniques (vide infra). The same synthesis took much longer with the reported clas-
sical routes [35–39]. The reaction of salicylaldehyde and diethylenetriamine in metha-
nol under reflux condition by classical method give 74% yield after 2 h. This clearly
demonstrated the advantage of grinding method over classical method.
The vanadium complex Na[VO2L] was synthesized by the LAG method by reacting
vanadyl sulfate and H2L. We used 0.25 mL of methanol/DMSO (1:1 mixture) as solvent
(Scheme 1) which is required to make the mixture pasty. The vanadium(IV) center
undergoes aerial oxidation to vanadium(V) during grinding. The presence of NaOH in
the reaction mixture is necessary as it provided the counter cation for the formation
of þ5 oxidation state in Na[VO2L]. The reaction was completed in 45 min of grinding
with isolated yield of 98%. We have also synthesized Na[VO2L] by refluxing the mix-
ture of vanadyl sulfate, NaOH and H2L in methanol. But the process required 24 h of
reaction time to complete. Thus, the LAG synthesis of Na[VO2L] is much more effect-
ive than the classical method.
3.2. Characterization
The Schiff base and Na[VO2L] were characterized with elemental analysis, EDX, FTIR
and NMR analyses. The elemental analysis data are presented in Table 1. Experimental

2060
J. J. BORUAH ET AL.
Table 1. Elemental analysis data for H2L and Na[VO2L].
Element
H2L (Expt./Theo.a)
Na[VO2L] (Expt./Theo.b)
C%
H%
N%
V%
69.04/69.43
6.48/6.80
13.22/13.49
52.43/52.06
4.11/4.61
9.79/10.12
12.06c/12.27
aCalculated for C18H21N3O2.
bCalculated for C18H19N3NaO4V.
cVanadium content was determined by volumetric methods as mentioned in Ref. [32].
data for C, H, N and V matched with the calculated theoretical values with respect to
the assigned molecular formulas. This shows the purity of the compounds in solid
form. The metal to ligand molar ratio was found to be 1:1 indicating incorporation of
one vanadium with one ligand. The EDX spectra showed the presence of C, N and O
in H2L and C, N, O, Na and V in Na[VO2L] (Figure 1). The magnetic susceptibility
measurement showed diamagnetic Na[VO2L] which confirmed the þ5 oxidation state.
Mass spectrometry showed the m/z peaks for [H2LþH]þ and [VO2LþNa þ H]þ at 312
and 416, respectively (Figure S1). These further confirmed the formation of H2L and
Na[VO2L]. The molar conductance of Na[VO2L] was 112 ohmꢀ1 cm2, indicating the
ionic nature of Na[VO2L] with ratio of cation to anion ¼ 1:1. Solution of H2L was
nonconductive.
The FTIR spectra for H2L and Na[VO2L] are shown in Figure 2 and their characteris-
tic peak assignments are given in Table 2. The spectrum of H2L showed a sharp peak
at 1627 cmꢀ1 which corresponds to ꢀ(C ¼ N)imine, confirming formation of the Schiff
base by condensation of salicylaldehyde and diethylenetriamine. The peak at
1627 cmꢀ1 in H2L shifted to 1620 cmꢀ1 in Na[VO2L]. This shifting of 7 cmꢀ1 after metal
incorporation confirmed the coordination of the vanadium(V) center with H2L through
the N of the imine group [35–40]. Additionally, the sharp broad peak at 3348 cmꢀ1 for
m(O-H) in H2L disappeared (or intensity reduced significantly) in Na[VO2L]. The peak at
1280 cmꢀ1 assigned to the phenolic ꢀ(C-O) in H2L shifted to 1296 cmꢀ1 in Na[VO2L].
These observations indicate coordination of the phenolic OH with the vanadium(V)
center [35, 41]. Newer sharp peaks at 1033 and 964 cmꢀ1 after metal incorporation
correspond to ꢀ(O ¼ V¼O)asym and ꢀ(O ¼ V¼O)sym, respectively. The presence of the
þ
asym and sym peaks for the ꢀ(O ¼ V¼O) further confirmed the presence of VO2 in
þ
the complex. The coordination of VO2 with the ligand was confirmed from the
appearance of two new peaks in the spectrum of Na[VO2L] at 630 and 451 cmꢀ1
,
attributable to m(V-O) and m(V-N), respectively [42, 43]. Thus, FTIR spectral analysis of
the synthesized compounds provided vital information as well as confirmed
the structure.
We have also calculated the IR peak positions by using GAUSSIAN16 program and
the values are shown in Table 2. The experimental and calculated values follow the
same trends, particularly ꢀ(C ¼ N)imine and ꢀ(C-O) peaks were red and blue shifted,
respectively, after metal coordination in both cases. Thus, the calculated IR peaks are
in agreement with the experimental values which strongly supported the formation of
Na[VO2L]. Notably, the calculated IR peak positions appeared at higher values than
the corresponding experimental data which may be due to the fact that the

JOURNAL OF COORDINATION CHEMISTRY
2061
Figure 1. EDX spectra of (a) H2L and (b) Na[VO2L].
Figure 2. FTIR spectra for (a) H2L and (b) Na[VO2L] (from 2000 to 400 cmꢀ1).

2062
J. J. BORUAH ET AL.
Table 2. FTIR Peak assignments for H2L and Na[VO2L].
Peak (cm–1
)
Compound
Experimental
Calculated
Assignment
m(O-H)
m(C ¼ N)imine
m(C-C)aromatic
m(CH2)
H2L
3348
1627
1581
1465
1280
3378
1683
—
—
1312
m(C-O)phenolic
Na[VO2L]
1620
1543
1450
1296
1033
964
1657
—
—
1342
1073
1026
687
492
m(C ¼ N)imine
m(C-C)aromatic
m(CH2)
m(C-O)phenolic
m(O ¼ V¼O)asym
m(O ¼ V¼O)sym
m(V-O)
630
451
m(V-N)
theoretical calculations are performed on a single molecule in the gas phase, while
the experimental data are from the solid state.
Electronic spectra of H2L and Na[VO2L] in DMSO are shown in Figure S2. The spec-
trum of H2L showed three absorptions at kmax ¼ 255, 314 and 404 nm. The first two
ꢄ
peaks at lower wavelength are attributed to p!p transition of the aromatic rings
ꢄ
and the low intensity peak at 404 nm is due to n!p transition of the C ¼ N group
[35, 44]. In the electronic spectrum of Na[VO2L], a new peak at 432 nm is assigned to
LMCT transition O(phenolic) ! V5þ [45–47]. The p!p transitions are red shifted to
ꢄ
261 and 324 nm after complexation. The shifting of peak position and appearance of
new peak in Na[VO2L] in comparison with the H2L indicated coordination of the lig-
and to the vanadium center [46].
1H NMR spectral analysis provides information about the formation of H2L and
Na[VO2L]. The data for each of the compounds along with peak assignments are pre-
sented in Table 3 (spectra are shown in Figure S3). There are differences in number of
peaks, their positions and patterns in each of the compounds. The singlet at 8.51 d
(2H) attributable to imine group confirmed the formation of Schiff base in H2L.
Further, the peak positions for aromatic (-C-H), methylene (-CH2-), phenolic (-OH) and
amine (-NH) groups were in their respective region as reported [48]. However, the
peak positions were shifted from the reported values for H2L [35, 37, 39] which may
be attributable to change in NMR solvent. After complex formation, the peak for phen-
olic -OH disappeared, indicating coordination of phenolic -OH group with the
vanadium(V) via oxygen. Upfield shifting of -HC ¼ N- proton in Na[VO2L] in compari-
son to H2L indicated N coordination of imine with metal in the complex. Thus 1H
NMR analysis confirmed the formation of H2L and Na[VO2L]. Moreover, the spectra of
both compounds were clean and void of unassignable peaks which indicated the pur-
ity of isolated products.
The proposed structure for the vanadium(V) complex, Na[VO2L], is shown in
þ
Scheme 1. Na[VO2L] consists of a dioxovanadium(V), VO2 moiety coordinated with
the ligand (H2L) through N and O center of the imine and hydroxy groups. The ligand
provided a four coordinating (N2O2-type) environment.

JOURNAL OF COORDINATION CHEMISTRY
2063
Table 3. 1H NMR Spectral data for H2L and Na[VO2L] (recorded in DMSO-d6).
Compound
Peak position (d)
Assignment
H2L
8.51(2H)
Imine, -HC ¼ N-
Aromatic, -C-H
Methylene, -CH2-
Phenolic, -OH
Amine, -NH
7.36(4H), 6.86(4H)
3.65(4H), 2.85(4H)
2.58(2H)
2.09(1H)
Na[VO2L]
8.38(2H)
Imine, -HC ¼ N-
Aromatic, -C-H
Methylene, -CH2-
Amine, -NH
7.31(4H), 6.80(4H)
3.58(4H), 2.77(4H)
2.13(1H)
Figure 3. Energies of the oxidation process mediated by H2O2.
3.3. Anti-bacterial activity of the compounds H2L and Na[VO2L]
The antibacterial activities of H2L and Na[VO2L] against Gram-positive and Gram-
negative bacterial strains are given in Table 4. We have chosen two Gram-positive (viz.
Staphylococcus aureus (S. aureus) and Bacillus subtilis (B. subtilis)) and two Gram-nega-
tive (Escherichia coli (E. coli) and Pseudomonas aeruginosa (P. aeruginosa)) bacterial
strains. The Schiff base H2L possess antibacterial activity against all the bacteria.
However, the vanadium(V) complex, Na[VO2L], possesses significantly higher antibac-
terial activity than H2L against all the microbial strains. The superior activity of the
complex over Schiff base can be explained on the basis of Overtone’s concept and
Tweedy’s chelation theory [49, 50]. We also compared the antibacterial activity of the
synthesized compounds with commercially available Ampicillin and vanadyl sulfate. It
was found that the activity of Na[VO2L] is 5–6% and 11–12% lower than Ampicillin
towards Gram-negative and Gram-positive bacteria, respectively. The metal precursor,
vanadyl sulfate, exhibited no zone of inhibition on Gram-negative bacteria under iden-
tical test conditions.
3.4. Catalytic activity: selective oxidation of alcohols and sulfides
The catalytic activity of Na[VO2L] was studied towards oxidation of organic alcohols
and sulfides. The reaction conditions such as reaction temperature, catalyst amount,
substrate:oxidant stoichiometry, solvents, microwave power, etc. were optimized by
using benzyl alcohol and methyl phenyl sulfide as representative substrates. Benzyl
alcohol may be oxidized into benzaldehyde and/or benzoic acid whereas methyl

2064
J. J. BORUAH ET AL.
Table 4. Anti-bacterial activities of H2L and Na[VO2L] on different bacteria.
Compound (inhibition zone diameter, mm)
Bacterial strain
Gram-negative
H2L
Na[VO2L]
VOSO4ꢁ5H2O
Ampicillin
E. coli
16
4
12
8
37
16
39
29
–
–
2
3
39
17
44
33
P. aeruginosa
S. aureus
B. subtilis
Gram-positive
- No zone of inhibition was observed.
Scheme 2. Oxidation products of methyl phenyl sulfide (1) and benzyl alcohol (2).
Scheme 3. Mechanism of catalytic oxidation by Na[VO2L] using methyl phenyl sulfide as substrate.
phenyl sulfide may be oxidized into methyl phenyl sulfoxide and/or methyl phenyl sul-
fone as shown in Scheme 2.
Na[VO2L] efficiently, catalytically and selectively oxidized benzyl alcohol into ben-
zaldehyde or benzoic acid with hydrogen peroxide as oxidant. The oxidation reactions
were solventless and achieved product selectivity by simply controlling the reaction
conditions. The use of 1.1 equivalent of hydrogen peroxide with respect to benzyl
alcohol produced benzaldehyde as sole product with high purity (Table 5). On the
other hand, use of 3.0 equivalent of hydrogen peroxide with respect to benzyl alcohol
produced benzoic acid as sole product. The reactions under microwave irradiation are
much faster than with classical heating methods. The selective oxidation of benzyl
alcohol to benzaldehyde was completed in 10 min of microwave irradiation whereas
the same reaction under heating condition needed 2.5 h to complete. No oxidation
took place in the absence of Na[VO2L] under identical reaction conditions. This indi-
cated that Na[VO2L] played a vital role in the oxidation process. We have extended
the catalytic alcohol oxidation activity of Na[VO2L] towards oxidation of 4-nitrobenzyl

JOURNAL OF COORDINATION CHEMISTRY
2065
Table 5. Catalytic oxidation of organic sulfides and alcohols by Na[VO2L] under microwave irradi-
ation and heating conditions.
Reaction condition
Microwavea
Heatingb
Substrate
Product
Time
10 minc
Yield (%)
Time
2.5 hc
Yield (%)
98
99
97
97
20 mind
17 minc
5.0 hd
4.0 hc
97
98
40 mind
98
7.0 hd
97
15 minc
35 mind
5 mine
96
97
98
3.5 hc
6.5 hd
97
98
97
20 mine
10 minf
10 mine
99
98
45 minf
90 mine
99
99
20 minf
5 mine
96
98
210 minf
25 mine
97
97
12 minf
96
60 minf
98
aAll reactions were conducted at 100 W power with 1.00 mmol substrate.
bAll reactions were conducted at 70 ꢃC with 0.5 mmol substrate.
cMolar ratio of substrate:H2O2:Na[VO2L] is 500:550:1.
dMolar ratio of substrate:H2O2:Na[VO2L] is 500:1500:1.
eMolar ratio of substrate:H2O2:Na[VO2L] is 500:525:1.
fMolar ratio of substrate:H2O2:Na[VO2L] is 500:1250:1.
and 4-chlorobenzyl alcohols and the results are summarized in Table 5. Na[VO2L] effi-
ciently and selectively oxidized the substrates under classical heating and micro-
wave methods.
We have also achieved the selective and catalytic oxidation of methyl phenyl sulfide
to methyl phenyl sulfoxide or methyl phenyl sulfone with Na[VO2L] under solventless

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J. J. BORUAH ET AL.
conditions. 100% conversion to methyl phenyl sulfoxide was found by applying 1.05
equivalent of hydrogen peroxide with respect to substrate and catalytic amount of
Na[VO2L]. The sulfoxide was isolated in pure form. On the other hand, use of 2.5
equivalent of hydrogen peroxide completely oxidized the methyl phenyl sulfide to
methyl phenyl sulfone with catalytic amount of Na[VO2L]. The oxidation reactions
were much faster under microwave irradiation than heating reactions. Under similar
reaction conditions, we have catalytically and selectively oxidized ethyl phenyl sulfide
and diphenyl sulfide into sulfoxide and sulfone. The results are summarized in Table 5.
3.5. Mechanism of catalytic oxidations
A plausible mechanism for catalytic oxidation by Na[VO2L] is shown in Scheme 3. The
dioxovanadium(V) complex Na[VO2L] is inactive, as such, in the oxidation reaction.
Na[VO2L] reacted with H2O2 to form oxoperoxovanadate(V) species II which is the
active species for oxidation of substrates (alcohol or thio ether). After oxidation, spe-
cies II converted back into Na[VO2L] and reacted with another molecule of H2O2 to
continue the catalytic cycle. The formation of catalytically active oxoperoxovanadate
species from Na[VO2L], in oxo-transfer, is well known and proposed on the basis of
available literature [51, 52].
3.6. Mechanism studied by computation
In order to establish the proposed mechanism (vide supra), we have done calculations
by using the GAUSSIAN16 suite of programs. The details of the computation method
are outlined in the experimental section and the energy of the oxidation process
mediated by H2O2 is shown in Figure 3. The first step is formation of vanadium peroxo
complex on treatment with H2O2 to the initial vanadium oxo complex. This step is
slightly endergonic by 5.4 kcal/mol. The peroxo O-O bond length is elongated
(1.425 Å). The peroxo complex then initiates the oxidation of PhSCH3 to the oxidized
product. This step is exergonic by 6.7 kcal/mol. Overall, the reaction is exergonic by
1.3 kcal/mol, thus, indicating the catalytic role of the vanadium complex towards
the oxidation.
4. Conclusion
We have achieved green synthesis of H2L and its vanadium(V) complex Na[VO2L] by
using the grinding method. The protocol is simple and required less reaction time to
complete than the conventional heating method. Both compounds were characterized
by elemental, mass, conductometric and spectroscopic analysis. The complex select-
ively and catalytically oxidizes methyl phenyl sulfide and benzyl alcohol under micro-
wave irradiation and conventional heating. The oxidation reactions under microwave
irradiation were much faster than the heating method. All the reactions were solvent-
less. A suitable mechanism for catalytic oxidation was proposed which was verified by
theoretical calculations using the GAUSSIAN16 suite. Na[VO2L] exhibited strong anti-
bacterial activity against Gram-positive and Gram-negative bacterial strains.

JOURNAL OF COORDINATION CHEMISTRY
2067
Acknowledgements
Authors are thankful to the Management, RK University, Rajkot, Gujarat, India for providing
financial support to procure chemicals and fees for analysis. We are also grateful to Department
of Microbiology, RK University, Rajkot, Gujarat, India for the help in study of anti-microbial activ-
ity. The authors take this opportunity to thank Central Salt and Marine Chemicals Research
Institute (CSMCRI), Bhavnagar, Gujarat, India and Centre of Excellence, NFDD Centre, Saurashtra
University, Rajkot, Gujarat, India for providing the analytical facilities.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
J.J.B. is thankful to the University Grants Commission, New Delhi, India for the financial support
under Minor Research Project (No. F. 5–30/2013–14/MRP/NERO/427).
ORCID
Siva Prased Das
http://orcid.org/0000-0002-4545-9857
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