Liquid-phase oxidation of olefins with rare hydronium ion salt of dinuclear dioxido-vanadium(V) complexes and comparative catalytic studies with analogous copper complexes

Article abstract of DOI:10.1002/aoc.6203

Homogeneous liquid-phase oxidation of a number of aromatic and aliphatic olefins was examined using dinuclear anionic vanadium dioxido complexes [(VO₂)₂(^salLH)]^? (1) and [(VO₂)₂(^NsalLH)]^? (2) and dinuclear copper complexes [(CuCl)₂(^salLH)]^? (3) and [(CuCl)₂(^NsalLH)]^? (4) (reaction of carbohydrazide with salicylaldehyde and 4-diethylamino salicylaldehyde afforded Schiff-base ligands [^salLH₄] and [^NsalLH₄], respectively). Anionic vanadium and copper complexes 1, 2, 3, and 4 were isolated in the form of their hydronium ion salt, which is rare. The molecular structure of the hydronium ion salt of anionic dinuclear vanadium dioxido complex [(VO₂)₂(^salLH)]^? (1) was established through single-crystal X-ray analysis. The chemical and structural properties were studied using Fourier transform infrared (FT-IR), ultraviolet–visible (UV–Vis), ¹H and ¹³C nuclear magnetic resonance (NMR), electrospray ionization mass spectrometry (ESI-MS), electron paramagnetic resonance (EPR) spectroscopy, and thermogravimetric analysis (TGA). In the presence of hydrogen peroxide, both dinuclear vanadium dioxido complexes were applied for the oxidation of a series of aromatic and aliphatic alkenes. High catalytic activity and efficiency were achieved using catalysts 1 and 2 in the oxidation of olefins. Alkenes with electron-donating groups make the oxidation processes easy. Thus, in general, aromatic olefins show better substrate conversion in comparison to the aliphatic olefins. Under optimized reaction conditions, both copper catalysts 3 and 4 fail to compete with the activity shown by their vanadium counterparts. Irrespective of olefins, metal (vanadium or copper) complexes of the ligand [^salLH₄] (I) show better substrate conversion(%) compared with the metal complexes of the ligand [^NsalLH₄] (II).

Full text of DOI:10.1002/aoc.6203

Received: 6 July 2020
DOI: 10.1002/aoc.6203
Revised: 18 September 2020
Accepted: 28 January 2021
F U L L P A P E R
Liquid-phase oxidation of olefins with rare hydronium ion
salt of dinuclear dioxido-vanadium(V) complexes and
comparative catalytic studies with analogous copper
complexes
Abhishek Maurya
| Chanchal Haldar
Department of Chemistry, Indian Institute
of Technology (Indian School of Mines),
Dhanbad, Jharkhand, 826004, India
Homogeneous liquid-phase oxidation of a number of aromatic and aliphatic
olefins was examined using dinuclear anionic vanadium dioxido complexes
[(VO2)2(salLH)]− (1) and [(VO2)2(NsalLH)]− (2) and dinuclear copper complexes
[(CuCl)2(salLH)]− (3) and [(CuCl)2(NsalLH)]− (4) (reaction of carbohydrazide
with salicylaldehyde and 4-diethylamino salicylaldehyde afforded Schiff-base
ligands [salLH4] and [NsalLH4], respectively). Anionic vanadium and copper
complexes 1, 2, 3, and 4 were isolated in the form of their hydronium ion salt,
which is rare. The molecular structure of the hydronium ion salt of anionic
dinuclear vanadium dioxido complex [(VO2)2(salLH)]− (1) was established
through single-crystal X-ray analysis. The chemical and structural properties
were studied using Fourier transform infrared (FT-IR), ultraviolet–visible
(UV–Vis), 1H and 13C nuclear magnetic resonance (NMR), electrospray ioniza-
tion mass spectrometry (ESI-MS), electron paramagnetic resonance (EPR)
spectroscopy, and thermogravimetric analysis (TGA). In the presence of hydro-
gen peroxide, both dinuclear vanadium dioxido complexes were applied for
the oxidation of a series of aromatic and aliphatic alkenes. High catalytic activ-
ity and efficiency were achieved using catalysts 1 and 2 in the oxidation of ole-
fins. Alkenes with electron-donating groups make the oxidation processes
easy. Thus, in general, aromatic olefins show better substrate conversion in
comparison to the aliphatic olefins. Under optimized reaction conditions, both
copper catalysts 3 and 4 fail to compete with the activity shown by their vana-
dium counterparts. Irrespective of olefins, metal (vanadium or copper) com-
plexes of the ligand [salLH4] (I) show better substrate conversion(%) compared
with the metal complexes of the ligand [NsalLH4] (II).
Correspondence
Chanchal Haldar, Department of
Chemistry, Indian Institute of Technology
(Indian School of Mines), Dhanbad
826004, Jharkhand, India.
Email: chanchal@iitism.ac.in
Funding information
Science and Engineering Research Board,
Grant/Award Numbers: SB/EMEQ-
055/2014, SB/FT/CS-027/2014
K E Y W O R D S
anionic vanadium complex, copper complex, homogeneous catalysis, hydronium ion salt, olefins
oxidation
Appl Organomet Chem. 2021;e6203.
wileyonlinelibrary.com/journal/aoc
© 2021 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.6203

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MAURYA AND HALDAR
1 | INTRODUCTION
olefins, the effectiveness of a transition metal catalyst
preferentially increases with the increasing oxidation
state of the metal. There are several factors responsible
for achieving the maximum oxidation state of the
transition metal in their complexes. Those are
(i) higher oxygen affinity, (ii) Lewis acid character of
the transition metal, (iii) easy interconvertible oxida-
tion states, and (iv) easy interchangeable coordination
numbers.[12]
Higher oxygen affinity, variable valances, and coor-
dination number make vanadium and copper com-
plexes an ideal choice for designing an oxidation
catalyst for the olefins. Hence, the proper design of a
multidentate Schiff-base ligand, along with the proper
choice of a transition metal, is the critical factor for the
development of a productive and powerful oxidation
catalyst. Based on the above discussion, it will be an
interesting observation to compare the catalytic activity
of vanadium and copper catalysts prepared by using the
same Schiff-base ligands for the alkene oxidation, which
may help us to identify the impact of metal centers on
the catalytic reaction.
There is a constant demand for various value-added
chemicals like alcohols, aldehydes, acids, esters, epoxides,
etc. in both industrial and chemical industries,[1] and it is
growing faster than ever. All these carbonyl compounds
and alcohols are traditionally used as solvents, perfumes,
and flavoring agents or essential intermediates for vari-
ous chemical feedstocks, polymers, and agrochemicals.
These carbonyl compound-based active intermediates are
used as primary materials for the production of dyes,
epoxy resins, paints, polymers, perfumes, pharmaceuti-
cals, plastics, and surfactants.[2] Conventionally these car-
bonyl compounds are produced by the oxidation of
olefins with the stoichiometric amount of conventional
oxidizing agents like permanganates, peracids, hydrogen
[3,4]
peroxides, or atmospheric O2.
However, they often
lead to low substrate conversion and the formation of
undesirable side products. Thus, transition metal-based
complexes appear like an alternative, green, and econom-
ical catalytic systems for the oxidation of olefins. A large
number of the report is evolving in the literature dealing
with the oxidation of olefins by various transition metal
(such as Ti, V, Cr, Mn, Co, Ni, Cu, Mo, Ru, and Re) based
catalysts in the presence of peroxides, peracids, and other
oxidizing reagents.[5–7] Among all the transition metals
mentioned above, vanadium and copper hold a unique
and popular choice in the field of oxidation catalysts of
various organic compounds.[8] Because of the high natu-
ral abundance and lower price, vanadium and copper
complexes are considered to be the most effective and
vital catalysts in the chemical industry. Moreover, they
are found to be actively involved in catalyzing the
alkene oxidation, epoxidation, sulfoxidation, alcohol oxi-
dation, oxidative bromination, oxidative couplings, and
condensations.[5b,6,9] Over the past decades, a vast num-
ber of vanadium and copper Schiff-base complexes have
been synthesized and applied for the oxidation of various
olefins.[2a,10]
However, the importance of the ligand environment
is unquestionable to tune the catalytic efficiency of a
catalyst. In this connection, Schiff-bases come across as
being an effective and prime ligand system. The pol-
ydentate nature of the Schiff-bases allows them to cre-
ate an impressive coordination sphere around the
transition metal, which is suitable for various applica-
tions such as catalysis, chemical analysis, photo-
physical studies, biological research, and optics.[11]
Also, Schiff-base complexes of transition metal gener-
ally improve the effectiveness of hydrogen peroxide by
converting it into metal peroxides during the alkene
oxidation. In the processes of catalytic oxidation of
Hence, in the current article, we have prepared two
hexadentate Schiff-base ligands [salLH4] (I) and [NsalLH4]
(II) with
N
and
O
donor atoms, by reacting
carbohydrazide with salicylaldehyde and 4-diethylamino
salicylaldehyde, respectively. The hexadentate Schiff-
bases were chosen in such a way that each ligand can
accommodate two metal centers simultaneously without
involving any extra bridging group/atoms in between the
metal centers. The restricted electronic distribution
between the two metal atoms of the same complex can
allow the individual metal atom to function as an inde-
pendent catalytic center. Herein, we report four binuclear
metal complexes [H3O][(VO2)2(salLH)] (1), [H3O]
[(VO2)2(NsalLH)] (2), [H3O][(CuCl)2(salLH)] (3), and
[H3O][(CuCl)2(NsalLH)] (4), which were synthesized
by reacting [salLH4] (I) and [NsalLH4] (II) with the metal
precursors of vanadium and copper. The ligand [salLH4]
(I) and its vanadium complex [NH4][(VO2)2(salLH)] (1)
can be found in earlier literature.[13] However, here, we
were able to crystallize complex 1 in the form of its
hydronium ion salt with 2 eq. of DMF as the solvent of
crystallization. First time we are introducing the vana-
dium and copper complexes [H3O][(VO2)2(NsalLH)] (2),
[H3O][(CuCl)2(salLH)] (3) and [H3O][(CuCl)2(NsalLH)]
(4). All the prepared vanadium and copper complexes
successfully catalyzed the oxidation of a range of alkenes
in the presence of hydrogen peroxide, a green oxidant.
However, vanadium complexes show their superior
behavior over copper-based catalysts for the alkene
oxidation.

MAURYA AND HALDAR
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2 | EXPERIMENTAL
2.1 | Materials
2.3 | X-ray crystallography
The single-crystal X-ray data of complex [H3O]
[(VO2)2(salLH)].2(CH3)2NCH (1) was collected by Rigaku
Oxford Diffraction system equipped with state of the art
CCD Eos S2 detector using MoKα radiation (wavelength
0.71073 Å) at room temperature. The structure was
solved by direct methods using SIR-2004[15] and refined
on F2 by full-matrix least square method using SHELXL-
2018.[16] The nonhydrogen atoms (C, O, N, and V) were
refined anisotropically, and hydrogen atoms were
included in the refinement on calculated positions riding
on their carrier atoms. The molecular graphics were
haggard using the PLATON program. The function
Salicylaldehyde
(SRL,
India),
4-diethylamino
salicylaldehyde (Alfa-Aesar, India), carbohydrazide (TCI,
Japan), CuCl2.2H2O (Loba Chemie, India), V2O5 (Merck,
India), acetylacetone (Merck, India), sulphuric acid
(Merck, India), potassium carbonate (Merck, India), 30%
H2O2 (Merck, India), 70% TBHP (Merck, India), styrene
(Alfa-Aesar, India), allylbenzene (Alfa-Aesar, India),
1-methyl cyclohexene (Alfa-Aesar, India), cis-cyclooctene
(Alfa-Aesar, India), 1-octene (Alfa-Aesar, India),
4-chlorostyrene (TCI, Japan), indene (TCI, Japan),
2-norbornene (TCI, Japan), limonene (TCI, Japan),
divinylbenzene (TCI, Japan), and AR grade solvents
(Merck & Rankem, India) were used as received. HPLC
grade methanol (Spectrochem, India), n-hexane
(Spectrochem, India), and acetonitrile (Spectrochem,
India) were used as received for GC analysis. VO(acac)2
was prepared by following the reported method.[14]
minimized was [Σw (Fo − Fc ) ] (w = 1/[σ2(Fo )
2
2 2
2
+ (aP)2 + bP]), where P = (Max (Fo ,0) + 2Fc )/3
2
2
with σ2(Fo ) from counting statistics. The function R1
2
2 2
and wR2 were (σjjFoj − jFcjj)/σjFoj and [σw (Fo2 − Fc ) /σ
(wFo )]1/2, respectively. CCDC No. 2009184 carries
4
the supplementary crystallographic data of complex 1 for
this paper. Detailed crystallographic data are enlisted in
Tables 1 and S1.
2.2 | Physical methods and analysis
3 | PREPARATIONS
The structure of ligands and metal complexes was char-
acterized by various physicochemical methods. Fourier
transform infrared (FT-IR) (4,000–400 cm−1) spectra of
ligands and metal complexes were recorded on an
Agilent Cary 600 Series FT-IR Spectrometers by the
ATR method. 1H nuclear magnetic resonance (NMR)
and 13C NMR spectra of the ligands were recorded in
DMSO-d6 solvent on a Bruker Avance II 400 MHz NMR
spectrometer with the general parameter settings.
Electronic spectra of ligands and metal complexes were
recorded in a “SHIMADZU” UV-1800 spectrophotome-
ter using DMF/methanol. The electrospray ionization
mass spectrometry (ESI-MS) of the metal complexes
were analyzed by Waters Q-Tof Micromass. Ther-
mogravimetric analysis (TGA) and differential thermal
analysis (DTA) were performed by using Perkin Elmer,
Diamond TG/DTA instrument. The X-band electron
paramagnetic resonance (EPR) measurements of copper
complexes were performed in a JES-FA200 ESR spec-
trometer. Catalytic oxidation of various alkenes was
monitored by an Agilent 7890B gas-chromatograph fitted
with an HP-5 capillary column (30 m × 0.25 mm
× 0.25 μm) and an flame ionization detector (FID). The
products of the catalytic reaction were identified by
Thermo GC–MS (Trace 1300 ISQ QD) with a TG-5MS
capillary column (30 m × 0.25 mm × 0.25 μm) and a
mass detector.
3.1 | Synthesis of hexadentate Schiff-
bases [salLH4] (I) and [NsalLH4] (II)
Ligands (I) and (II) were prepared by adopting the
methods reported in the literature[17] and presented in
Scheme 1. Briefly, the methanolic solution of (10 ml)
carbohydrazide (0.90 g, 0.01 mol) was reacted with
salicylaldehyde (2.44 g, 0.02 mol) or 4-diethylamino
salicylaldehyde (3.86 g, 0.02 mol) under the refluxing
condition for 5–6 h. The volume of the resulting reaction
mixture was reduced to ꢀ5 ml. After keeping the solution
at room temperature, the white-colored precipitate was
separated and washed with cold methanol followed by
petroleum ether and dried under vacuum over silica gel.
3.1.1 | Data for [salLH4] (I)
Yield: 62.78%; Anal. Calcd. For C15H14N4O3
(MW 298.30); C, 60.40%; H, 4.73%; N, 18.78%; found: C,
60.53%; H, 4.81%; N, 18.65%; FT-IR (ATR, cm−1): 3,413
(νO H), 3,254 (νN H), 1,708 (νC O), 1,609(νC N);
ultraviolet–visible
(UV–Vis)
[λmax
(nm),
ε
(L mol−1 cm−1)]: 231 (3.46 × 103), 250 (Sh) (2.89 × 103),
341 (Sh) (2.02 × 103), 384 (2.84 × 103), 454 (3.28 × 103);
1H NMR (DMSO-d6, δ in ppm): 6.8–6.8 (m, 4H), 7.1–7.2

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MAURYA AND HALDAR
TABLE 1 Crystal and refinement parameters for complex
[H3O][(VO2)2(salLH)].2(CH3)2NCH (1).
(m, 2H), 7.6 (s, 2H), 8.3 (s, 2H), 10.5 (s, 2H), 10.8 (s, 2H);
13C NMR (DMSO-d6, δ in ppm): 116.2, 119.2, 119.7,
128.1, 130.7, 147.2, 152.0, 156.6.
Identification code
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
a (Å)
(1)
C21H28N6O10V2
626.37
3.1.2 | Data for [NsalLH4] (II)
293 (2)
Triclinic
P-1
Yield: 72.45%; Anal. Calcd. For C23H32N6O3
(MW 440.54); C, 62.71%; H, 7.32%; N, 19.08%; found: C,
62.43%; H, 7.53%; N, 18.94%; FT-IR (ATR, cm−1): 3,398
(νO H), 3,260 (νN H), 1,699 (νC O), 1,627 (νC N); UV–Vis
[λmax (nm), ε (L mol−1 cm−1)]: 218 (2.95 × 103),
249 (1.43 × 103), 337 (Sh) (4.48 × 103), 367 (7.32 × 103);
1H NMR (DMSO-d6, δ in ppm): 1.0–1.1 (t, 12H), 3.2–3.3
(q, 8H), 6.1–6.1 (m, 2H), 6.2–6.2 (m, 2H), 7.3–7.3 (m, 2H),
8.2 (s, 2H), 10.3 (s, 2H), 10.6 (s, 2H); 13C NMR (DMSO-
d6, δ in ppm): 12.5, 43.8, 97.6, 103.6, 107.4, 130.1, 144.4,
149.6, 152.1, 158.5.
9.2398 (16)
11.2699 (14)
13.584 (2)
89.473 (11)
74.417 (14)
76.228 (13)
1,321.1 (4)
2
b (Å)
c (Å)
α (ꢁ)
β (ꢁ)
γ (ꢁ)
Volume (Å3)
Z
ρcalc (g cm−3
)
1.575
μ (mm−1
)
0.772
3.2 | Synthesis of [H3O][(VO2)2(salLH)]
(1) and [H3O][(VO2)2(NsalLH)] (2)
F(000)
644.0
Radiation Mo Kα (λ)
0.71073
A common route was followed for the preparation of the
complexes 1 and 2. Briefly, a methanolic solution of
[VO(acac)2] (5.30 g, 0.02 mol) was reacted with methanolic
solution of [salLH4] (I) (2.98 g, 0.01 mol) or [NsalLH4] (II)
(4.40 g, 0.01 mol) under refluxing condition for 2 h. The
resultant reaction mixture was kept for slow evaporation
overnight. During this time, a green-colored precipitate of
1 and an orange-colored precipitate of 2 separated out
from the reaction mixture. It was filtered, washed with hot
methanol followed by petroleum ether, and dried in a
vacuum under silica gel. Crude samples of 1 and 2 were
dissolved in the mixed solvents (DMF and MeOH) and
kept for crystallization. Within 6–7 days, yellowish colored
crystals of 1, suitable for single-crystal X-ray analysis, were
filtered off, washed with cold methanol, and dried in air.
Despite our continuing efforts, we were unable to get the
crystals of 2 suitable for single-crystal X-ray analysis.
2θ range for data
3.728 to 51.994
collection (ꢁ)
Index ranges
−11 ≤ h ≤ 10, −13 ≤ k ≤ 13,
−13 ≤ l ≤ 16
Reflections collected
9,948
Independent reflections
5,069 (Rint = 0.1411,
Rsigma = 0.2025)
Data/restraints/
parameters
Goodness of fit on F2
5,069/0/368
0.930
Final R indexes R1(I > 2σ
(I)), wR2
R1 = 0.1040, wR2 = 0.2120
Final R indexes (all data)
R1, wR2
R1 = 0.1896, wR2 = 0.2554
0.95/−0.63
Largest diff. peak/hole/e
Å−3
SCHEME 1 Synthetic scheme for the
preparation of ligands [salLH4] (I) and [NsalLH4]
(II) along with their corresponding vanadium
(1–2) and copper (3–4) complexes

MAURYA AND HALDAR
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3.2.1 | Data for [H3O][(VO2)2(salLH)] (1)
3.3.2 | Data for [H3O]
[(CuCl)2(NsalLH)] (4)
Yield: 55.20%; Anal. Calcd. For C15H14N4O8V2
(MW 480.18): C, 37.52%; H, 2.94%; N, 11.67%; found: C,
37.93%; H, 2.85%; N, 11.02%; FT-IR (ATR, cm−1): 3,347
(νO H), 3,212 (νN H), 1,675 (νC O), 1,591 (νC N), 914, 932
Yield: 55.98%; MW 654.54 (C23H32Cl2Cu2N6O4), FT-IR
(ATR, cm−1): 3,338 (νO H), 3,197 (νN H), 1,660 (νC O),
1,600 (νC N); UV–Vis [λmax (nm), ε(L mol−1 cm−1)]:
213 (1.63 × 103), 224 (1.64 × 103), 238 (1.14 × 102),
296 (1.45 × 102), 308 (1.21 × 102), 661 (143); ESI-MS:
(νV O); UV–Vis [λmax (nm),
ε
(L mol−1 cm−1)]:
242 (1.71 × 103), 367 (1.38 × 103), 439 (9.62 × 102),
525 (5.05 × 102); electrospray ionization mass spectrome-
try (ESI-MS): m/z 461.93 (shown in Figure S13, calculate
for [(VO2)2(salLH)]− = 461.15).
m/z
=
735.05
(calculated
{[(CuCl)2(NsalLH)].
735.63); m/z = 601.24 (shown
2H2O.2MeOH}
=
in Figure S16, calculated [{[Cu2(NsalLH)].2H2O} + H]
= 601.64).
3.2.2 | Data for [H3O][(VO2)2(NsalLH)] (2)
3.4 | Catalytic oxidation of alkenes
Yield:
58.01%;
Anal.
Calcd.
C23H32N6O8V2
The catalytic efficiency of all the synthesized complexes
1, 2, 3, and 4 were evaluated towards the catalytic oxida-
tion of alkenes. All the reactions were performed in a
50-ml round-bottom flask fitted with a water-cooled con-
denser. In a typical reaction, styrene (0.520 g, 5 mmol)
was reacted with 30% H2O2 (1.133 g, 10 mmol) in the
presence of 0.003 g of catalyst precursor in 5 ml of MeOH
with continuous stirring at 70ꢁC of bath temperature for
6 h. A small portion of the reaction mixture was with-
drawn periodically and extracted with n-hexane and ana-
lyzed by GC. The substrate conversion(%) and product
selectivity(%) were calculated by using Equations 1 and 2.
To find out the maximum substrate conversion(%) and
product selectivity(%), reaction conditions were opti-
mized by changing various parameters, that is, catalyst
amount, oxidant amount, solvent amount, solvent
nature, nature of oxidant, and reaction temperature.
(MW 622.42); C, 44.38%; H, 5.18%; N, 13.50%; found: C,
45.04%; H, 5.25%; N, 13.72%; FT-IR (ATR, cm−1): 3,371
(νO H), 3,203 (νN H), 1,675 (νC O), 1,585 (νC N), 908, 944
(νV O); UV–Vis [λmax (nm),
ε
(L mol−1 cm−1)]:
214 (6.18 × 103), 348 (4.09 × 103), 411 (2.81 × 103); ESI-
MS: m/z 603.11 (shown in Figure S14, calculate for
[(VO2)2(NsalLH)]− = 603.40).
3.3 | Synthesis of [H3O][(CuCl)2(salLH)]
(3) and [H3O][(CuCl)2(NsalLH)] (4)
By adopting similar methods as described earlier for the
preparation of vanadium complexes 1 and 2, we have pre-
pared complexes 3 and 4. Briefly, a methanolic solution
of CuCl2.2H2O (3.40 g, 0.02 mol) was mixed with a
methanolic solution of [salLH4] (I) or [NsalLH4] (II) in 2:1
ratio with constant stirring under the refluxing condition
for 2 h. The resultant reaction mixture was cooled and
kept in a refrigerator overnight. Within a day, a greenish
colored precipitate of 3 and 4 was filtered off, washed
with cold methanol, and dried in a vacuum under
silica gel.
%Conversion of Substrate−100
ð1Þ
Area of substrate
−
× 100,
Total area of substrate + Area of products
Similarly,%Selectivity = 100
ð2Þ
Area of product
−
× 100:
Total area of products
3.3.1 | Data for [H3O][(CuCl)2(salLH)] (3)
4 | RESULTS AND DISCUSSION
4.1 | X-ray crystallography
Yield: 68.78%; MW 512.29 (C15H14Cl2Cu2N4O4), FT-IR
(ATR, cm−1): 3,392 (νO H), 3,209 (νN H), 1,675 (νC O),
1,600 (νC N); UV–Vis [λmax (nm), ε (L mol−1 cm−1)]:
222 (2.64 × 103), 276 (1.66 × 103), 392 (8.96 × 102),
639 (70.21); ESI-MS: m/z = 423.03 (shown in Figure S15,
calculated {[(Cu2(salLH)] + H} = 423.36).
The molecular structure of the complex [H3O]
[(VO2)2(salLH)].2(CH3)2NCH (1) has been confirmed by
single-crystal X-ray diffraction (XRD) analysis. The single

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MAURYA AND HALDAR
crystals of 1 are quite sensitive to moisture. If kept for a
long time in the air at room temperature, they start to
lose their crystallinity and convert into powder, probably
by losing the solvent of crystallization. The complex
1 crystalized in the triclinic crystal system with P-1 space
group where each vanadium adopts distorted square
pyramidal geometry. The ORTEP plot of 1 is displayed in
Figure 1.
The asymmetric unit of the crystal structure of the
molecule 1 carries an anionic binuclear vanadium com-
plex with two molecules of DMF (solvent of crystalliza-
tion) and one hydronium ion as a counter cation. A
limited number of reports are available in the literature
where hydronium ion was incorporated in the molecule
to balance the charges.[18a,b]
strong and directional intermolecular H-bonding
(Figure 2). One of the dioxido vanadium(V) oxygen
involves in the intermolecular hydrogen bonding with
the amine hydrogen of the neighboring molecule and
vice versa (D H …. A = 2.936 Å). A detailed list of H-
bond distances is tabulated in Table S2. Each delicate
hydronium ion is holding two O atoms from two DMF
solvent molecules, one dioxido vanadium(V) oxygen, and
one phenolate oxygen atom through H-bonding. These
H-bondings are sufficiently strong enough to hold and
stabilized the hydronium ion inside the crystal packing of
1. Also, a weak interaction can be observed between one
of the hydronium ion hydrogen and the neighboring
dioxido vanadium(V) complex (Figure 2b).
Trianionic hexadentate carbohydrazide Schiff-base
ligand (HsalL3−) captures two vanadium centers with a
mutual distance of 5.007 Å, where each vanadium center
is present as cis-dioxido form. Interestingly, both cis-VO2
unit of each molecule is mutually trans-directed to each
other (Figure 1b). The geometrical index τ is 0.23 for 1,
which (τ = 0 for ideal square pyramidal and τ = 1 for per-
fect trigonal bipyramidal geometry) describes the extent
of distortion in the geometry around the metal cen-
ters.[18c] Interestingly, the electronic environment around
both vanadium centers is not similar. The basal plane
around the V1 is occupied by oxygen (O1) of phenolate
group, azomethine nitrogen (N1) atom, and nitrogen
(N3) from the carbohydrazide group, whereas V2 is sur-
rounded by phenolate oxygen (O4), azomethine nitrogen
(N4), and carbonyl oxygen (O5) of carbohydrazide group.
The C8 O5 bond length is 1.264 Å, which is a little lon-
ger than the >C O double bond length but significantly
shorter than the C O single bond indicating that the O5
is a carbonyl O atom. The important bond angles and
bond lengths of 1 are tabulated in Table S1.
4.2 | Spectroscopic analysis
A range of spectroscopic techniques was utilized to
understand as well as establish the molecular structure of
the prepared ligands and corresponding metal (vanadium
and copper) complexes.
FT-IR spectroscopy was used to record the changes in
functional groups in every step of the metal complex syn-
thesis, starting from their respective ligands systems (I)
and (II). Ligands (I) and (II) carry an FT-IR signal at
1,609 and 1,627 cm−1, respectively, which is the charac-
teristic bands of νC N, suggesting the successful forma-
tion of the ligand systems. A sharp and strong stretching
band due to νC
appears at 1,708 (I) and 1,699 (II),
O
cm−1, which implies the existence of the ligands in the
keto-form in the solid state. Additionally, both ligands
display a broad signal for νO
in the range of
in the range of
H
3,398–3,413 cm−1 and νN
H
3,254–3,260 cm−1. The FT-IR spectra of ligands and their
metal complexes are displayed in Figures S1–S6. Selected
stretching frequencies are tabulated in Table S3. Vana-
dium complexes 1 and 2 produce a couple of splitted
Molecular association in the solid state of 1 is con-
trolled by a number of weak interactions, dominated by
FIGURE 1 (a) ORTEP plot of the asymmetric unit in the crystal of complex [H3O][(VO2)2(salLH)].2(CH3)2NCH (1) with 50% thermal
ellipsoids and (b) polyhedral plot of 1

MAURYA AND HALDAR
7 of 21
FIGURE 2 (a) Crystal packing 1 in a 3D structure in solid state and (b) intermolecular H-bonding involving hydronium [H3O]+ ion
vibrational band characteristics of νsym(O V O) and
νasym(O V O) of V(V)O2 species, in the range of
914–932 cm−1 and 908–944 cm−1, respectively.[5a,19] In
both copper complexes 3 and 4, stretching bands of νC
O
and νC
appear a little shifted towards the lower
N
wavenumber compared with their corresponding ligand,
which suggests the coordination of “O” of keto group and
“N” atom of azomethine group to the copper center.[5a]
Successful metalation of the ligands [salLH4] (I) and
[
NsalLH4] (II) were further supported by the electronic
spectroscopic analysis. UV–Vis spectra of ligands and
their corresponding metal complexes were recorded in
methanol and displayed in Figure S7. All the four metal
complexes 1–4 show comparable electronic spectra with
that of the free ligands. The vanadium complexes 1 and
2 exhibit an additional band in the range of 390–411 nm,
which is assigned to phenolate “O” to vanadium(V)
charge transfer transition. On the other hand, both cop-
per complexes 3 and 4 displayed one low-intensity broad
band in the range of 639–661 nm, characteristic of the d-
d transition of Cu2+ center.[2c,20] The above-mentioned
changes in the absorbance spectra of the metal complexes
support the successful complexation. A list of important
λmax values along with the molar extinction coefficients
(ε) are tabulated in Table S4. The gradual addition of a
2-drop portion of 0.1 M 30% H2O2 solution into a
FIGURE 3 Spectral changes recorded by the gradual addition
of two drops of 0.1M H2O2 solution in the 3.22 × 10−2 M DMSO
solution of [(CuCl)2(salLH)]− (3)
in the range of 6.8–7.2 ppm. A similar spectral pattern
can be seen in the proton NMR spectra of the ligand (II)
in the range of 6.1–7.3 ppm. Both ligands display a sharp
singlet characteristic of azomethine proton at 8.3 (I) and
8.2 ppm (II). As expected, no aliphatic proton signal is
visible in the ligand (I). A sharp triplet with 12 equivalent
protons centered at 1.0 ppm due to the methyl ( CH3,
Ј = 6.9 Hz) groups and a quartet with 8 equivalent pro-
tons centered at 3.3 ppm, because of a methylene group
1
methanolic solution of
3
(3.67
×
10−4 M) and
( CH2 , J = 6.68, 6.88 Hz), can be seen in the H NMR
4 (5.60 × 10−4 M) causes the disappearance of 639 nm (3)
and 661 nm (4) bands respectively (shown in Figures 3
and S8), suggesting the conversion of Cu2+ into Cu3+ spe-
cies in solution.
spectra of (II). Ligand (I) exhibits the characteristic
NH, and OH proton signal at 10.5 and 10.8 ppm,
respectively. As both the signals partially merge, the
sharper signal is assigned to the less labile amine protons,
whereas the broader signal is due to the labile OH pro-
tons. Comparable spectra can be observed in the ligand
(II), but alcoholic ( OH) protons appear more des-
hielded than the amine protons, which may be due to the
After getting the initial assessment from the FT-IR
and UV–Vis spectroscopy, the molecular structure of the
ligands [salLH4] (I) and [NsalLH4] (II) was confirmed
through NMR spectroscopy. The proton NMR spectra of
the ligands [salLH4] (I) and [NsalLH4] (II) are displayed in
Figure 4, and the appropriate assignment of proton sig-
nals is given in Table S5. Ligand [salLH4] (I) shows the
three aromatic proton signals of eight equivalent protons
1
stronger intramolecular H-bonding. H NMR spectra of
both ligands (I) and (II) were re-recorded in the presence
1
of D2O. Comparative analysis of H-NMR spectra of (I)
and (II) recorded in the presence of D2O and without

8 of 21
MAURYA AND HALDAR
FIGURE 4 Comparative proton NMR spectra of ligands [salLH4] (I) and [NsalLH4] (II) recorded in DMSO-d6 and in DMSO-d6 + D2O
FIGURE 5 Comparative proton NMR spectra of 1 recorded in the presence and absence of D2O using DMSO-d6 as a base solvent

MAURYA AND HALDAR
9 of 21
D2O showed that in the presence of D2O, proposed alco-
holic ( OH) and amine ( NH) signals disappear along
with slight shifting in the signals of aromatic and ali-
phatic protons. The absence of proton signals in the
range of 10.3–10.8 ppm further confirms the OH and
NH protons.
original chemical shift values. The NH proton was
1
further confirmed through H 1H COSY, and the spec-
trum is shown in Figure S9. All the diagonal peaks
1
resemble what we observed in the H NMR spectra of 1.
Two very weak cross-peaks appear at {7.9, 8.8} and
{7.9, 6.8} ppm due to the magnetic interaction of one of
the azomethine protons with amine and phenylic pro-
tons, respectively. These observations confirm the pres-
ence of NH proton in 1, and the chemical shift suggests
that this amine group is not coordinated to the metal
center.
In the 1H NMR spectrum of 1 (as shown in Figure 5),
the signals due to the eight aromatic protons appear in
the range of 6.8–7.6 ppm. A sharp singlet equivalent to
two protons, due to the azomethine protons ( HC N), is
present at 7.9 ppm. A singlet because of one amine pro-
ton ( NH) is found at 9.0 ppm. Due to the presence of
two molecules of DMF as solvent of crystallization in 1,
we observed two singlets in the range of 8.6–9.7 ppm aris-
ing from the aldehydic protons. Additionally, two sin-
glets, equivalent to six protons each in the range of
2.7–2.8 ppm, were also observed due to the methyl
groups of DMF molecules. A broad singlet at 11.2 ppm
can also be seen due to the OH of hydronium ion. The
chemical shift of amine proton was determined by re-
The molecular frame of ligands [salLH4] (I) and
[
NsalLH4] (II) was further established by 13C NMR spectra
and is shown in Figure 6. The appropriate assignment of
signals is tabulated in Table S6. Due to the symmetrical
molecular arrangements, 13C NMR spectra of ligands (I)
and (II) show eight and ten non-equivalent carbon sig-
nals, respectively. The characteristic carbonyl carbon
appears in (I) at 156.6 ppm, whereas (II) displays it at
158.5 ppm. Azomethine carbon signal appears at 147.2
(in I) and 149.6 ppm (in II). Except for the phenolic car-
bon atom, all the other aromatic carbon signals appear in
their expected region. Additionally, ligand (II) exhibits
two aliphatic carbon signals, one at 12.5 ppm, due to
the CH3 groups, another at 43.8 ppm, because of the
CH2 groups. All signals are in accordance with the
molecular structure predicted by proton NMR and FT-IR
spectroscopy.
1
recording H NMR spectrum of the DMSO-d6 solution of
1 after adding a few drops of D2O and compared with the
1
original H NMR spectrum (recorded in the presence of
DMSO-d6 only). Figure 5 shows that the addition of D2O
causes the disappearance of the 9.0- and 11.2-ppm signal,
which confirms that the signals at 9.0 and 11.2 ppm are
due to the NH and OH proton, respectively. The aro-
matic and aliphatic signals show slight shifting from their
FIGURE 6 13C NMR spectra of ligands [salLH4] (I) and [NsalLH4] (II) recorded in DMSO-d6

10 of 21
MAURYA AND HALDAR
FIGURE 7 (a) X-band EPR spectra of
[(CuCl)2(salLH)] (3) and [(CuCl)2(NsalLH)] (4)
recorded in DMF at liquid nitrogen temperature,
(b) the expanded gk region of the 3 and 4
X-band EPR spectra of the copper complexes [H3O]
4.3 | Thermal analysis (TGA/DTA
[(CuCl)2(salLH)] (3) and [H3O][(CuCl)2(NsalLH)] (4) were
recorded at liquid nitrogen temperature in DMF which
are presented in Figure 7. It is interesting to note that
complexes 3 and 4 generate well-resolved hyperfine split-
ting pattern in the parallel region with four groups of
peaks with an average splitting of 111 and 112 G, respec-
tively. Close observation reveals (Figure 7b) that each
group of peaks is comprised of two closely associated
peaks.
This is because of the fact that two differently posi-
tioned but similar types of copper(II) nuclear spin
(I = 3/2) interact with the spin of its unpaired electron
(I = 1/2), and their hyperfine structures mutually overlap
with each other. Hence, recorded EPR spectra imply the
presence of two magnetically different copper(II) centers
in the molecules 3 and 4. Other groups have reported
similar types of magnetically different dicopper(II) cen-
ters present in the same molecule where the spin–spin
exchange interaction among the copper(II) centers was
observed.[21] The axially symmetrically EPR spectra of
the complexes 3 and 4 do not produce any forbidden
analysis)
The thermal stability of the metal complexes 1–4 was
examined by TGA/DTA method under the nitrogen
atmosphere at a temperature rate of 10ꢁC/min over a
temperature range of 30–915ꢁC and displayed in
Figure 8.
Initially, all four metal complexes 1–4 experience a
minor mass loss due to the elimination of entrapped sol-
vents, as evident from the single-crystal XRD analysis of
1. Later on, they follow the multistep breakdown process
where partial decomposition of ligands or metal com-
plexes starts. From the TGA/DTA analysis, it is clear that
the complexes 1–4 are thermally stable up to 211ꢁC,
241ꢁC, 290ꢁC, and 190ꢁC, respectively. Hence, complexes
1–4 can be easily applied as a catalyst under moderate
temperature without breaking their molecular integrity.
4.4 | Catalytic oxidation of alkenes
Δ mS
=
2 transition at the half field, which rules
After a thorough characterization of vanadium and cop-
per complexes 1–4, their catalytic aptitude was examined
and compared for the oxidation of alkenes. Oxidation of
alkene produces oxygenated functionalization in the mol-
ecule, which is extremely important for the production of
fine chemicals. Catalytic oxidation of alkenes was scruti-
nized using styrene and vanadium-based complex [H3O]
[(VO2)2(salLH)] (1) as a model substrate and catalysts,
respectively. Catalytic oxidation of styrene in the
out the possibility of spin coupling between the two
copper(II) centers through the ligand frame.
The calculated principal components of g tensor for
3 are g = 2.3998 and g┴ = 2.0895, whereas complex
4 shows g = 2.4044 and g┴ = 2.0889 (listed in the
Table 2). The EPR spectra of both copper(II) complexes
k
k
show g > g┴ > ge, which is the characteristic feature of
k
a distorted square planar metal center.[21b,22]
TABLE 2 Data of spine Hamilton
parameter of 3 and 4 in DMF solution
[H3O][(CuCl)2(salLH)] (3)
[H3O][(CuCl)2(NsalLH)] (4)
Property
A
k
A
k
at LNT.
g
2.3998
2.0895
111 G
2.4044
2.0889
112 G
k
g┴

MAURYA AND HALDAR
11 of 21
of substrate conversion was achieved by using 1:1 sub-
strates to oxidant molar ratio (Figure 9b), whereas 1:2
and 1:3 produce comparable substrate conversion(%).
Slight decline in the substrate conversion can be seen
while using 1:4 substrates to oxidant molar ratio. In order
to optimize the volume of solvent during the catalytic oxi-
dation of styrene, four different volumes of methanol,
that is, 2.5, 5, 7.5, and 10 ml were tested while keeping
all the other parameters fixed. The corresponding effects
are shown in Figure 9c. With a rise in the volume of sol-
vent from 2.5 to 10 ml, substrate conversion(%) decreases
gradually due to the reduction of the effective collision
among the reactant molecules in a larger volume of sol-
vent. A small reduction (2%) in the substrate conversion
(%) is observed while increasing the solvent amount from
2.5 to 5 ml. However, 5 ml volume was chosen as opti-
mum to avoid the inherent errors associated with the
handling of the smaller volume of the solvent.
FIGURE 8 TGA plot of [H3O][(VO2)2(salLH)] (1), [H3O]
[(VO2)2(NsalLH)] (2), [(CuCl)2(salLH)] (3), and [(CuCl)2(NsalLH)] (4)
presence of hydrogen peroxide produces mainly benzoic
acid and 1-phenylethane-1,2 diol, along with a small
amount of acetophenone and benzaldehyde (shown in
the Scheme 2), which are commonly reported in the
literature.[2c,6,23]
The measurable reaction parameters were precisely
optimized to attain the maximum substrate conversion
(%) by varying the reaction parameters, namely, catalyst
amount, oxidant amount, solvent amount, nature of sol-
vents, types of oxidants, and temperature. Moreover, the
impact of the metal center on the catalytic oxidation of
alkenes was checked by using the copper counterpart of
catalyst 1 under similar optimized conditions.
Four different quantities, i.e., 0.041, 0.125, 0.208, and
0.291 mol% of catalyst were screened to optimize the
effect of catalyst (1) amounts on the oxidation of styrene
while using a fixed amount of styrene (0.520 g, 5 mmol)
and 30% H2O2 (1.700 g, 15 mmol) in 5 ml of methanol at
60ꢁC for 6 h. The outcomes are shown in Figure 9a.
While increasing the quantity of catalyst from 0.041 g to
0.125 mol%, a massive improvement in substrate conver-
sion(%) (from 29% to 86%) was observed. However, a fur-
ther rise in the catalyst amount does not increase the
substrate conversion(%) much. Hence, 0.125 mol% of cat-
alyst was considered as optimum.
The nature of the solvent has a strong influence on
the catalytic oxidation of styrene in the presence of
hydrogen peroxide. Three different types of solvents,
DMF, CH3CN, and MeOH, were used as a reaction
medium while keeping other parameters the same. The
impact of the nature of solvents is displayed in Figure 9d.
A
maximum substrate conversion(%) of 85% was
achieved in methanol. Nevertheless, with decreasing the
polarity of the solvent, substrate conversion decreases,
and hence, in acetonitrile, 42% of conversion was
achieved. However, only 2% conversion was observed in
DMF, which may be due to the higher distribution ratio
of the oxidation products in DMF.
Similarly, to choose the suitable oxidizing agents, oxi-
dation of styrene was executed with two different oxi-
dants, such as 30% H2O2 and TBHP, and the result is
shown in Figure 9e. TBHP shows only 36% conversion,
whereas up to 85% conversion can be achieved with
hydrogen peroxide. Mainly two factors are responsible for
the lower activity of TBHP. One, during the catalytic
reaction, metal complexes 1–4 reacts with the available
peroxide to form metal peroxides, which are responsible
for the alkene oxidation, and the metal peroxides which
are generated in the presence of TBHP are relatively wea-
ker in nature in comparison to the hydrogen peroxide
drove metal peroxides. Two steric bulkiness can cause
trouble for an easy substrate association with the metal
center in case of TBHP containing metal peroxides.
The influence of oxidant amount on oxidation reac-
tion was studied by using four different substrates to oxi-
dant molar ratios, namely, 1:1, 1:2, 1:3, and 1:4. Only 51%
Finally, the influence of bath temperature was
studied by monitoring the styrene oxidation in five
different bath temperatures, 50ꢁC, 55ꢁC, 60ꢁC, 65ꢁC,
and 70ꢁC (shown in Figure 9f). With the rising bath
temperature, substrate conversion(%) increases gradually,
and a maximum of 94% conversion was achieved at 70ꢁC.
Hence, 70ꢁC of bath temperature was considered as
SCHEME 2 Oxidation products of styrene catalyzed by 1–4 in
the presence of hydrogen peroxide at 70ꢁC bath temperature

12 of 21
MAURYA AND HALDAR
FIGURE 9 Optimization of catalytic oxidation of styrene with the catalyst [H3O][(VO2)2(salLH)] (1): (a) effect of catalyst amount,
(b) effect of 30% H2O2 amount, (c) impact of solvent amount, (d) influence of nature of the solvent, (e) influence of nature of oxidant, and (f)
effect of temperature
optimum. The final optimized reaction conditions are
0.125 mol% of catalyst 1, H2O2 as an oxidant, MeOH as a
solvent, oxidant:substrate ratio of 1:2, 5 ml of solvent,
70ꢁC bath temperature, and 6 h of reaction time (listed in
entry no. 17 of Table 3).
evident from the data that copper catalysts 3 and 4 are
not as efficient as their vanadium counterparts in terms
of substrate conversion(%) or efficiency. Catalyst 3 dis-
plays 41% conversion with TOF values of 28.50 h−1
,
whereas 37% conversion with TOF value of 33.81 h−1 is
shown by catalyst 4. With more than 50% product selec-
tivity, benzoic acid (in methanolic solution it converts
into methyl benzoate) appears as a major product in
every case.
Under the above-optimized reaction conditions, cata-
lyst 2 shows comparable substrate conversion (93%) to
that of the catalyst 1 (94%). However, in terms of effi-
ciency, catalyst 2 shows a little higher turnover frequency
(TOF) (= 80.61 h−1
)
values compared with cat
The blank reaction of styrene oxidation without cata-
lyst shows only 5% conversion. Moreover, in the presence
of the metal precursors, VO(acac)2 and CuCl2, oxidation
of styrene was monitored and displayed in Figures 10
and 11. Data confirm that styrene oxidation under
1 (TOF = 62.76 h−1). To observe the impact of metal cen-
ters in the catalytic reaction, corresponding copper cata-
lysts 3 and 4 were also applied for the styrene oxidation
under the same optimized reaction conditions. It is

MAURYA AND HALDAR
13 of 21
TABLE 3 Data for all the parameters applied to optimize for maximum conversion of styrene oxidation by catalyst [H3O]
[(VO2)2(salLH)] (1).
S.N.
1
Mole % of Cat.
0.0416
0.125
Temp. (ꢁC)
Sub.:Oxi.
1:3
Oxidant
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
TBHP
H2O2
H2O2
H2O2
H2O2
Solvent (ml)
Solvent
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
DMF
% Conv.
29
TOF (h−1
)
60
60
60
60
60
60
60
60
60
60
60
60
60
50
55
65
70
5
5
59.62
57.88
36.41
27.26
34.41
57.22
48.75
59.15
47.08
44.28
1.40
2
1:3
86
3
0.208
1:3
5
91
4
0.291
1:3
5
95
5
0.125
1:1
5
51
6
0.125
1:2
5
85
7
0.125
1:4
5
73
8
0.125
1:2
2.5
7.5
10
5
88
9
0.125
1:2
70
10
11
12
13
14
15
16
17
0.125
1:2
66
0.125
1:2
2
0.125
1:2
5
AcCN
42
28.41
24.14
55.28
56.62
58.75
62.89
0.125
1:2
5
MeOH
MeOH
MeOH
MeOH
MeOH
36
0.125
1:2
5
82
0.125
1:2
5
84
0.125
1:2
5
88
0.125
1:2
5
94
Abbreviation: TOF, turnover frequency.
FIGURE 10 Comparison of % conversion of
blank reaction with the catalysts and their
respective metal salts under optimized reaction
conditions
optimized reaction conditions in the presence of metal
precursors VO(acac)2 and CuCl2 proceeds with 60% and
28% substrate conversion, respectively. So, this implies
that the vanadium and copper centers are catalytically
active and essential for the styrene oxidation. However,
while using catalysts 1–4, a substantial improvement of
substrate conversion(%) is visible in comparison to the
metal precursors. It justifies that not only metal centers,
but the associated ligand frame is equally important and
necessary for the efficient function of the catalyst. Hence,
the current ligand system is boosting the styrene oxida-
tion efficiency of the vanadium as well as the copper
centers. The data suggest that in comparison to the
vanadium-based catalysts 1–2, copper-based catalysts are
quite less effective (except 1-octene) towards styrene oxi-
dation in terms of substrate conversion and efficiency
(Table 4). The plausible reason for the lower efficiency of
copper catalysts 3–4 in comparison with their vanadium
counterparts is because vanadium makes stronger metal
peroxides than copper, which are the active species
responsible for the catalytic oxidation.
We extended our study to the oxidation of a number
of aromatic and aliphatic alkenes, and the results are
summarized in Table 4. Each alkene was oxidized in the

14 of 21
MAURYA AND HALDAR
FIGURE 11 Comparison of % conversion of
blank reaction with the catalysts and their
respective metal salts under optimized reaction
conditions
presence of catalysts 1–4, and the obtained conversion(%)
was compared with the metal precursor catalyzed reac-
tions as well as the blank reactions. Comparison of sub-
strate conversion(%) achieved in the presence of the
catalysts 1–4 and their metal precursors along with the
blank reactions is plotted and displayed in Figures 10
and 11.
Some interesting aspects of alkene oxidation can be
observed while analyzing the data acquired in the pres-
ence of catalysts 1–4. Broadly, two classes of compounds,
that is, aromatic and aliphatic alkenes, were used for
the experimental purpose. Out of the listed 10 olefins,
five are aromatic, and five are aliphatic. Under the
optimized reaction conditions, in general, aromatic ole-
fins show higher substrate conversion(%) in the presence
substrate conversion for other aliphatic alkenes in the
presence of catalyst 1 is 2-norbornene (88%) > 1-octene
(68%)
> cis-cyclooctene (62%) > limonene (50%).
Catalyst 2 delivers the same order of reactivity with
the listed aromatic alkenes, but aliphatic alkenes show
a different order of reactivity than the catalyst 1,
which is, 1-methylcyclohex-1-ene (88%) > 2-norbornene
(78%) > limonene (47%) > cis-cyclooctene (39%) >
1-octene (32%). It seems that electronic factors no longer
controlled the reactivity pattern in aliphatic alkenes,
especially among limonene, cis-cyclooctene, and
1-octene. A second factor, probably the steric effect
cooperation with electronic factor decides the reactivity
pattern of these alkenes in the presence of catalysts
1 and 2.
of vanadium-based catalysts
1
and 2. Styrene,
While analyzing the data of oxidation of alkenes
using copper-based catalysts 3 and 4, it appears that the
second factor, that is, steric bulkiness, contributes more
than the electronic factor in deciding the reactivity of
listed alkenes towards oxidation. Hence, the listed
alkenes come out more random in the reactivity pattern.
Styrene and cis-cyclooctene show higher substrate con-
version(%) among the aromatic and aliphatic alkenes,
respectively. Irrespective of alkenes, [NsalLH4] (II)-based
vanadium and copper catalysts perform less effectively in
comparison with the [salLH4] (I)-based vanadium and
copper catalysts. The presence of electron-donating group
N,N-diethylamine at the p-position in the ligand (II)
leads to the formation of weaker metal peroxo complexes
compared to the ligand (I)-based metal peroxo com-
plexes, and it is evident from the catalytic data that the
latter metal peroxo complexes (ligand (I)-based com-
plexes) outperform the former ones (ligand (II)-based
complexes). Detailed product selectivity of the oxidation
of various alkenes is summarized in Table 5.
divinylbenzene, and indene are showing comparable sub-
strate conversion (>94%). 4-Chlorostyrene shows 83%
conversion, whereas the least conversion(%) is observed
in allyl benzene (44%). In olefins like styrene,
divinylbenzene, and indene, the ethylenic double bond is
electron rich because of its conjugation with the phenyl
ring. Therefore, these olefins can easily participate in the
oxidation process. Although in 4-chlorostyrene, the
presence of electron-withdrawing group ( Cl) at the
p-position of the phenyl group depletes the electron
density from the ethylenic double bond, thereby making
them less active towards oxidation purpose. On the other
hand, in allyl benzene, there is no conjugation between
the π-electrons of the phenyl ring and the π-electrons of
the ethylenic double bond. Hence, it acts like an aliphatic
alkene and makes it least active among all the listed
aromatic alkenes. Thus, electron-rich alkenes undergo
oxidation reaction easily in the presence of vanadium-
based catalysts 1 and 2. By similarly considering the
electronic effect, among all the studied aliphatic alkenes,
1-methylcyclohex-1-ene shows the highest substrate con-
version (91%) in the presence of catalyst 1. The order of
It should be noted that in the methanolic solution, no
carboxylic acid was detected due to the conversion of car-
boxylic acids into the corresponding methyl esters. In

MAURYA AND HALDAR
15 of 21
TABLE 4 Conversion(%) and TOF values of oxidation of various alkenes using all the catalysts (1–4) in the presence of H2O2 in 5 ml
MeOH at 70ꢁC temperature.
S.N.
Cat.
Substrate
% conversion
TOF (h-1)
62.89
Reference
[29]
1
1
2
3
4
1
2
3
4
94
93
41
37
50
47
25
21
80.74
29.59
33.81
[30,31]
2
33.94
41.32
18.00
19.36
[29d,32]
[33,34]
[35]
3
4
5
6
1
2
3
4
1
2
3
4
1
2
3
4
1
2
3
4
94
92
37
34
62
39
27
25
44
38
20
16
83
81
36
36
62.75
80.13
26.89
31.63
41.88
34.06
19.35
22.90
29.67
33.19
14.51
14.54
55.95
70.19
26.25
33.09
[2b]
[27,36]
7
8
9
1
2
3
4
1
2
3
4
1
2
3
4
91
88
25
23
68
32
70
54
88
78
26
21
60.95
76.67
18.35
21.54
45.68
27.92
50.09
49.81
59.08
67.42
19.06
19.18
[2b,29d,32,37]
[27,32]
(Continues)

16 of 21
MAURYA AND HALDAR
TABLE 4 (Continued)
S.N.
Cat.
Substrate
% conversion
TOF (h-1)
63.02
Reference
10
1
2
3
4
94
93
14
12
80.48
10.03
10.99
Abbreviation: TOF, turnover frequency.
TABLE 5 Result of oxidation of olefins catalyzed by 1–4
S.N.
Cat.
% con.
TOF (h-1)
% selectivity of products
1
1
2
3
4
94
93
41
37
62.89
80.74
29.59
33.81
10.6
12.4
22.9
30.5
7.8
11.2
22.4
17.4
59.5
55.9
50.1
46.9
21.9
20.2
4.4
5.0
2
1
2
3
4
94
93
14
12
63.02
80.48
10.03
10.99
15.7
16.1
22.8
21.3
7.7
7.2
23.9
17.3
17.6
18.9
52.5
59.1
19.3
21.1
40.0
38.5
3
1
2
3
4
44
38
20
16
29.67
33.19
14.51
14.54
26.9
23.7
26.2
32.9
52.4
55.1
37.4
36.4
6.6
7.4
13.9
13.5
24.2
21.6
12.0
8.8

MAURYA AND HALDAR
17 of 21
TABLE 5 (Continued)
S.N.
Cat.
% con.
TOF (h-1)
% selectivity of products
4
1
2
3
4
94
92
37
34
62.75
80.13
26.89
31.63
62.6
80.0
25.8
30.6
5.5
3.6
63.7
58.3
31.9
28.4
18.5
23.2
32.7
28.8
13.5
21.1
5
1
2
3
4
91
88
25
23
60.95
76.67
18.35
21.54
8.5
9.3
9.5
9.1
20.6
23.5
33.4
28.4
17.9
12.9
33.2
27.8
52.8
54.0
23.8
34.4
6
1
2
3
4
68
32
70
54
45.68
27.92
50.09
49.81
38.5
39.0
43.4
50.9
32.0
35.0
27.2
26.0
29.4
25.8
15.7
12.4
—
—
13.6
10.5
7
—
1
2
3
4
83
81
36
36
55.95
70.19
26.25
33.09
9.3
11.4
19.2
20.0
51.8
61.4
71.3
68.6
38.8
27.1
9.4
—
—
—
—
—
11.3
8
(Continues)

18 of 21
MAURYA AND HALDAR
TABLE 5 (Continued)
S.N.
Cat.
1
% con.
TOF (h-1)
% selectivity of products
88
59.08
12.5
4.1
83.3
—
2
3
4
78
26
21
67.42
19.06
19.18
9.4
9.5
3.1
38.2
34.1
86.9
52.2
55.1
—
—
—
—
10.7
9
1
2
3
4
50
47
25
21
33.94
41.32
18.00
19.36
33.8
41.2
17.3
18.7
23.1
25.0
17.4
14.4
51.7
48.0
33.9
37.8
—
—
—
—
—
—
10
1
2
3
4
62
39
27
25
41.88
34.06
19.35
22.90
92.3
97.3
65.0
67.3
7.6
2.6
—
—
—
—
—
—
—
—
34.9
32.6
Note. Products selectivity(%), substrate conversion(%), and TOF value observed under optimized reaction conditions.
Abbreviation: TOF, turnover frequency.
other solvents, such as acetonitrile, the carboxylic acid
can be detected.
Even though the reaction mechanism of the catalytic
oxidation of alkenes was not studied in the present work,
a number of reports can be found in the literature[2c,24–26]
discussing the detailed mechanism of similar systems.
Based on our electronic spectroscopic studies and earlier
reports, it is anticipated that in the first step, the metal
complex converts into the peroxo species by reacting with
hydrogen peroxide. This active metal peroxo species goes
back to its native form by oxidizing the available alkenes.
We did not detect the active metal peroxo complexes
directly. The gradual addition of 1 drop portion of 0.1 M
H2O2 into the methanolic solution of [H3O]
[(VO2)2(salLH)] (1) complex (6.99 × 10−4 M) causes the
disappearance of the 390-nm band, while a new band
generates at 320 nm, and the 280 nm band intensifies
without changing its λmax (shown in Figure 12). All these
FIGURE 12 Spectral changes observed during the progressive
addition one drop of 0.1M H2O2 in the 6.99 × 10−4 M methanolic
solution of [H3O][(VO2)2(salLH)] (1) complex

MAURYA AND HALDAR
19 of 21
changes indicate the formation of [(VO2)2(salLH)]-
peroxido species in solution.[6a,27,28] In the presence of
properly diluted H2O2, complexes 2–4 also exhibit the
electronic spectral changes characteristic of metal peroxo
species in solution (shown in Figures S10–S12). The d-d
transitions of complexes 3 and 4 disappear by incremen-
tal addition of 0.1 M 30% H2O2 solution into a
055/2014) of the work. A. M. is thankful to IIT (ISM)
Dhanbad for fellowship. The authors would like to thank
SAIF Mumbai for TGA/DTA and X-band EPR analysis.
The authors acknowledge CFR IIT (ISM) Dhanbad for
providing single-crystal XRD analysis. The authors also
acknowledge SAIF Punjab University for ESI-MS data.
The authors would like to thank DST-FIST (project
no. SR/FST/CSI-256) for 400 MHz NMR facility in the
department of chemistry IIT (ISM) Dhanbad. GC used in
this study was procured from the grant given by Science
and Engineering Research Board (SERB), Department of
Science and Technology (DST) (grant no SB/FT/CS-
027/2014), Government of India, New Delhi, India, to
C. H. The authors would like to thanks Dr. Nikita
Chaudhary for helping in writing the manuscript. Special
thanks to Dr. H. P. Nayek, Department of Chemistry for
the X-ray crystallographic studies.
3.67
×
10−4
M
methanolic solution of
3
and
5.60 × 10−4 M methanolic solution of 4, which further
supports the formation of Cu3+-peroxo species in the
solution.[2c,22c,29c] Spectral changes have been shown in
Figures 3 and S8.
5 | CONCLUSION
Two unique and rare bimetallic vanadium complexes
[H3O][(VO2)2(salLH)] (1) and [H3O][(VO2)2(NsalLH)] (2)
were successfully synthesized. Molecular structures and
properties of the complexes 1 and 2 were thoroughly ana-
lyzed via a number of spectroscopic and thermal studies.
Single-crystal X-ray analysis was used to confirm the pro-
posed molecular structure of complex 1 in solid-state.
Both the vanadium centers are in the +5 oxidation state
in complexes 1 and 2. Bimetallic vanadium complexes
1 and 2 act as an effective catalyst for the oxidation of
several aromatic and aliphatic alkenes. It was observed
that electron-rich alkenes are easy to oxidize; hence, in
general, aromatic alkenes with electron donor groups are
performing better than the electron-deficient aliphatic
alkenes. To investigate the effect of the metal center in
the alkene oxidation, copper complexes [H3O]
[(CuCl)2(salLH)] (3) and [H3O][(CuCl)2(NsalLH)] (4) were
synthesized using the same ligand systems I and II which
were used for the preparation of vanadium complexes
and characterized in detail. Surprisingly, copper-based
catalysts 3 and 4 act less efficiently towards the oxidation
of the alkenes under the same optimized reaction condi-
tions set for the vanadium complexes. Furthermore, the
ligand [NsalLH4] (II) with an electron-donating group
makes relatively weaker oxidation catalysts as compared
with [salLH4] (I). Therefore, irrespective of alkenes, bime-
tallic complexes 1 and 3 overperform 2 and 4. Although
the performance of copper-based catalysts is not very
impressive for the listed alkenes, their vanadium counter-
parts are extremely effective and efficient in comparison
with the currently available vanadium-based homoge-
neous catalysts.
AUTHOR CONTRIBUTIONS
Abhishek Maurya: Conceptualization; investigation.
Chanchal Haldar: Data curation; formal analysis;
funding acquisition; methodology; project administra-
tion; resources; software; supervision; validation;
visualization.
CONFLICT OF INTEREST
There are no conflicts of interest to declare.
SUPPLEMENTARY INFORMATION
Additional data on single-crystal XRD, FI-IR, UV–Vis,
1H, and 13C NMR, and catalytic activity can be found in
the supporting information.
DATA AVAILABILITY STATEMENT
The data that support the findings of this study are avail-
able in the supplementary material of this article.
ORCID
Abhishek Maurya https://orcid.org/0000-0001-6977-
9532
Chanchal Haldar https://orcid.org/0000-0003-4642-
7918
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How to cite this article: Maurya A, Haldar C.
Liquid-phase oxidation of olefins with rare
hydronium ion salt of dinuclear dioxido-vanadium
(V) complexes and comparative catalytic studies
with analogous copper complexes. Appl Organomet
Chem. 2021;e6203. https://doi.org/10.1002/
aoc.6203