Substituent effects in dioxovanadium(V) schiff-base complexes: Tuning the outcomes of oxidation reactions

Article abstract of DOI:10.1016/j.poly.2021.115268

Dioxovanadium(V) salicylaldehyde semicarbazone complexes with substituents on the ligand that span the range of electron donating (methoxy) to electron withdrawing (nitro) have been synthesized and characterized by NMR, IR, CV and EPR. The reactivity of these complexes toward the oxidation of styrene (as compared to the proteo complex and vanadyl acetylacetonate) has been studied in the presence of two different oxidants (hydrogen peroxide and tert-butyl hydrogen peroxide, TBHP). The complexes have been shown to exhibit different selectivity towards epoxidation versus oxidative cleavage based on the substitution of the ligand and the oxidant chosen. Epoxidation is favored with the methoxy substituted complex in the presence of hydrogen peroxide, while oxidative cleavage is the preferred reaction pathway for the nitro substituted complex with hydrogen peroxide. Conversions for these reaction are comparable to similar catalysts but with improved selectivity.

Full text of DOI:10.1016/j.poly.2021.115268

Polyhedron 205 (2021) 115268
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Polyhedron
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Substituent effects in dioxovanadium(V) schiff-base complexes: Tuning
the outcomes of oxidation reactions
a
a
b
Vanessa P. McCaffrey a, , Olivia Q. Conover , Michael A. Bernard , Jonathan T. Yarranton ,
⇑
Nicholas R. Lessnau a, Jordan P. Hempfling a
a Department of Chemistry, Albion College, Albion MI 49224, USA
b Department of Chemistry, Michigan State University, East Lansing MI 48824, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Dioxovanadium(V) salicylaldehyde semicarbazone complexes with substituents on the ligand that span
the range of electron donating (methoxy) to electron withdrawing (nitro) have been synthesized and
characterized by NMR, IR, CV and EPR. The reactivity of these complexes toward the oxidation of styrene
(as compared to the proteo complex and vanadyl acetylacetonate) has been studied in the presence of
two different oxidants (hydrogen peroxide and tert-butyl hydrogen peroxide, TBHP). The complexes have
been shown to exhibit different selectivity towards epoxidation versus oxidative cleavage based on the
substitution of the ligand and the oxidant chosen. Epoxidation is favored with the methoxy substituted
complex in the presence of hydrogen peroxide, while oxidative cleavage is the preferred reaction path-
way for the nitro substituted complex with hydrogen peroxide. Conversions for these reaction are com-
parable to similar catalysts but with improved selectivity.
Received 28 February 2021
Accepted 12 May 2021
Available online 16 May 2021
Keywords:
Vanadium
Schiff-base ligands
Oxidation
Oxidative cleavage
Epoxidation
Ó 2021 Elsevier Ltd. All rights reserved.
1. Introduction
Previous work has shown that substituents on the ligand frame-
work of dioxovanadium(V) complexes can affect the reactivity of
Vanadium compounds have been shown to be useful catalysts
in the functionalization of alkanes and alkenes.[1] Complexes with
oxidovanadium and oxovanadium cores in high oxidation states
are particularly effective in the oxidation of alkanes, alkyl aromat-
ics, vinyl compounds, sulfide containing compounds, and alcohols.
[2] The use of these complexes in the conversion of alkenes to
epoxides and carbonyl compounds is of particular synthetic signif-
icance[3] and there has been much work done on optimizing the
conditions required for epoxidation using a variety of ligand frame-
works.[4,5,6] Table 1.
However, in many of these reports, oxidative cleavage of the
alkene moiety is listed as an undesirable side product of the reac-
tion. While these CAC bond cleaving reactions can be a drawback
in many synthetic applications, the optimization of these reactions
have potential use in the valorization of lignin to produce fine
chemical feedstocks.[7,8] Vanadium-containing catalysts have a
great deal of potential in this area due to the relatively mild condi-
tions that are required for these transformations and therefore
there is potential for greener and more sustainable chemistry using
these types of catalysts.[9,10]
the complex in a variety of biological applications[11,12,13] and
synthetic transformations.[14,15] Complexes based on Schiff-base
ligands are popular for these types of studies due to their ease of
synthesis and the wide variety of building blocks available for
use as starting materials. In particular, salicylaldehyde based
frameworks are particularly useful in structure–activity relation-
ship studies due to the ability to select substituents from strongly
electron donating alkoxy groups (-OR) to the strongly withdrawing
nitro moiety (–NO2) to dial in the desired electronic properties of
the ligand. To this end, we have synthesized a series of dioxovana-
dium(V) salicylaldehyde semicarbazone complexes (Scheme 1) to
investigate how altering the electron density within the ligand
framework affects the product outcomes in the oxidation of styr-
ene. The substituents were chosen so as to span the range of the
strongly electron donating (OCH3) to the strongly electron with-
drawing (NO2) to sample the largest variation of electron density
within the ring.
2. Materials and Methods
All common laboratory chemicals were purchased from com-
mercial sources and used without further purification, unless
otherwise noted.
⇑
Corresponding author.
E-mail address: vmccaffrey@albion.edu (V.P. McCaffrey).
https://doi.org/10.1016/j.poly.2021.115268
0277-5387/Ó 2021 Elsevier Ltd. All rights reserved.

V.P. McCaffrey, O.Q. Conover, M.A. Bernard et al.
Polyhedron 205 (2021) 115268
Table 1
Selected FTIR vibration bands for the new complexes and their associated ligands (cmÀ1).
Compound
v(C@O)
v(C@N)
(OAH)
v(CAO)
vs(VO+2)
vas(VO+2)
1a
3a
1b
3b
1695
1681
1669
1660
1600
1597
1553
1551
3100
3015
N/A
1569
1493
1550
1545
N/A
N/A
910
889
N/A
N/A
845
835
N/A
Scheme 1. Synthesis of dioxovanadium(V) semicarbazone complexes
(1H, NH, s), 11.54 (1H, Ar-OH, bs). 13C NMR (DMSO-D6, ppm),
d = 110.1, 117.4, 117.6, 121.6, 137.2, 150.4, 152.9, 157.2
Characterization Methods: H and C NMR spectra of the free
ligands and complexes were recorded on a 400 MHz Jeol spectrom-
eter in DMSO d6 (DMSO = dimethyl sulfoxide). IR spectroscopic
data were recorded on a Bruker ALPHA FT-IR spectrophotometer
as solid samples using a diamond attenuated total reflectance
(ATR) accessory. cw-EPR experiments were performed at X band
with a Bruker E-680X spectrometer. Electrochemistry was per-
formed using a CH Instruments potentiostat in a 0.1 M tetrabuty-
lammonium hexafluorophosphate (TBAPF6) DMSO solution with
a Pt working electrode, Pt counter electrode, and a Ag wire
pseudo-reference electrode in an Ar-filled glovebox. Cyclic voltam-
metry measurements were conducted with a 100 mV/s scan rate.
All potentials were internally referenced to the Fc/Fc+ redox couple.
TBAPF6 was purchased from Oakwood Chemical Company and
recrystallized from ethanol twice prior to use. GC/MS analyses
were performed using an Agilent 6890 series GC System equipped
with a HP-5MS column and Agilent 5973 Network Mass Selective
Detector. The column (30 m  0.25 mm ID and film thickness of
5-Methoxysalicylaldehyde Semicarbazone, 3a: Semicarbazide
hydrochloride (0.0739 g, 0.663 mmol), 5-methoxysalicylaldehyde
(0.10 g, 0.657 mmol) in 10 mL methanol, and sodium acetate
(0.0892 g, 0.656 mmol) in 15 mL methanol. The reaction mixture
was sealed and placed in the refrigerator overnight. A white solid
was recovered by filtration (0.0945 g, 69%). C9H10O5N3V (291.1):
calc. C 37.1, H 3.5, N 14.4. found C 36.3, H 3.5, N 13.9. 1H NMR
(DMSO-d6, ppm), d = 3.71 (3H, OCH3, s), 6.62 (2H, NH2, bs), 6.76
(2H, 2Ar-H, s), 7.35 (1H, Ar-H, s), 8.10 (1H, N = CH, s), 9.38 (1H,
Ar-OH, bs), 10.18 (1H, NH, s). 13C NMR (DMSO-d6, ppm),
d = 116.9, 122.4, 126.1, 134.6, 140.8, 160.8, 161.9, 169.8.
Dioxovanadium(V) Complex Synthesis. Synthesis of 2b was per-
formed using a published procedure.[11] 1b and 3b were synthe-
sized
by
dissolving
equimolar
amounts
of
vanadyl
acetylacetonate and the respective ligand separately in minimum
volumes of ethanol. These were combined and then brought to
reflux in air for 24 h. After cooling, an orange-yellow solid formed.
The precipitate was isolated with vacuum filtration and washed
with ice-cold ethanol.
0.25 lm, Agilent) was used with helium as the carrier gas at a con-
stant flow rate of 1.0 mL/min. The injector was held at 225 °C. Data
were acquired and processed with HP-Chemstation software.
Dioxovanadium(V) 5-Nitrosalicylaldehyde Semicarbazone, 1b:
Vanadyl acetylacetonate (0.2992 g, 1.1 mmol) and 5-nitrosalicy-
laldehyde semicarbazone (0.2546 g, 1.1 mmol). A gold-yellow pre-
cipitate (0.2871 g, 84%) was collected and analyzed by NMR and IR.
Attempts to grow single crystals suitable for X-ray diffraction were
unsuccessful. 1H NMR (DMSO-D6, ppm), d = 6.82 (1H, Ar-H, d), 7.68
(2H, NH2, bs), 8.13 (1H, Ar-H, d), 8.50 (1H, Ar-H, s), 8.53 (1H,
N = CH, s), 10.40 (1H, NH, bs).
Dioxovanadium(V) 5-Methoxysalicylaldehyde Semicarbazone, 3b:
Vanadyl acetylacetonate (0.3280 g, 1.2 mmol) and 5-methoxysali-
cylaldehyde semicarbazone (0.2541 g, 1.2 mmol). An orange solid
(0.1341 g, 38%) was collected and analyzed by NMR and IR.
Attempts to grow single crystals suitable for X-ray diffraction
resulted in microcrystalline samples that were not able to be ana-
lyzed. 1H NMR (DMSO-D6, ppm), d = 3.74 (3H, OCH3, s), 7.98 (2H,
NH2, bs), 6.75 (1H, Ar-H, d), 7.04 (1H, Ar-H, dd), 7.08 (1H, Ar-H,
d), 8.55 (1H, N = CH, s).
Syntheses of the ligands. All reactions were performed under air
using standard glassware. Vanadyl acetylacetonate (VO(acac)2)[16]
and salicylaldehyde semicarbazone[17] were prepared according
to published literature procedures. 1a and 3a were synthesized
by dissolving equimolar amounts of the substituted salicylalde-
hyde starting material, semicarbazide hydrochloride and sodium
acetate separately in minimum volumes of warm methanol and
then combining. The mixture was allowed to react for 24 h, and
the resulting solid collected using vacuum filtration. The ligands
were characterized by 1H NMR, 13C NMR, and IR.
5-Nitrosalicylaldehyde Semicarbazone, 1a: Semicarbazide
hydrochloride (0.9149 g, 8.2 mmol) and 5-nitrosalicylaldehyde
(1.3666 g, 8.2 mmol) and sodium acetate (1.1137 g, 8.2 mmol).
Upon combining the semicarbazide and 5-nitrosalicylaldehyde, a
precipitate formed. More solid formed upon addition of the sodium
acetate. The reaction was brought to reflux for 24 h. After workup,
a fluffy yellow solid (1.5617 g, 85%) was obtained. C8H7O6N4VÁ2H2-
O (342.1): calc. C 28.1H 3.2, N 16.4. found. C 27.9, H 2.8, N 16.1. 1H
NMR (DMSO-D6, ppm), d = 6.62 (2H, NH2, bs), 7.00 (1H, Ar-H, d),
8.05 (1H, Ar-H, d), 8.15 (1H, Ar-H, s), 8.69 (1H, N = CH, s), 10.39
Oxidation reactions: All reactions were run twice with a 100:1 M
ratio of styrene (or other starting material) to vanadium(V) cata-
lyst. The oxidant (30% aqueous hydrogen peroxide or 70% aqueous
2

V.P. McCaffrey, O.Q. Conover, M.A. Bernard et al.
Polyhedron 205 (2021) 115268
tert-butyl hydrogen peroxide) was in a 2:1 molar ratio of oxidant to
starting material. The dioxovanadium(V) catalyst was dissolved in
acetonitrile (25 mL) and heated to 80 °C. Styrene (0.51 g, 4.9
mmols) was added dropwise. The reaction mixture was main-
tained at 80 °C with stirring. The oxidant (30% aq. HOOH or 70%
aq. tert-butyl hydrogen peroxide) was added via micropipette at
time = 0. Aliquots were taken at 30, 60, 120 min, 4 h and 6 h and
were analyzed using GC/MS.
The
3
complexes under investigation were characterized
using reductive electrochemistry. The traces were ill defined
with no reversible behavior that could be attributed to the vana-
dium center. The CV of 3b showed multiple peaks, all of which
were irreversible, with the first reduction occurring at À1.56 V
(Fig. 2). The irreversibility of the redox behavior of these com-
plexes is similar to what has been seen for other dioxovanadium
complexes.[21] Similarly for the 2b, there were multiple irre-
versible reductive events with the first reduction occurring at
À1.47 V. The shift to more negative potentials compared to the
proteo is in agreement with the increased electron-donation by
3. Results
the methoxy group into the ligand-based
p-system. The nitro
3.1. Synthesis and Characterization
substituted complex which had first irreversible reduction at
À0.96 V and a reversible couple at À1.62 V (the latter is attrib-
uted to reduction of the dianion of uncomplexed 1a).[22] The
more positive reduction potential of the nitro-substituted ligand
is in agreement with the increased electron-withdrawing ability
of the substituent.
Two new dioxovanadium(V) semicarbazone complexes (1b and
3b) have been synthesized through established synthetic routes
(shown in Scheme 1) and characterized by NMR, IR and electro-
chemical means. The two new complexes have the formulae of
VO2L, where L is 5-nitrosalicylaldehyde semicarbazone or 5-
methoxysalicylaldehyde semicarbazone. The complexes were syn-
thesized in good yields with high purity and the spectroscopic
results support the proposed structures. The spectroscopic proper-
ties of the unsubstituted complex (2b) were identical to those that
have been published previously.[11]
The complexes are soluble in DMSO and basic aqueous solu-
tions, moderately soluble in acetonitrile but only sparingly soluble
in ethanol, methanol and other organic solvents. All attempts to
grow single crystals of the 5-methoxy and 5-nitro complexes were
unsuccessful. The recovered solids were either microcrystalline or
powdery, and therefore, structural determination was based on
other spectroscopic means.
The NMR spectra showed clean conversion of the ligands into
the complexes, with no non-complexed ligand as contaminants.
The NMR signals for the complexes were sharp signals with little
broadening, suggesting that the complex was primarily in the
vanadium(V) oxidation state. The 1H NMR spectrum of the 5-nitro
complex showed strong signals and all proton resonances were
accounted for. However, while the spectrum of the 5-methoxy
complex showed sharp peaks, the intensity of the signal was very
low and didn’t increase when the sample was concentrated. The
spectrum was also missing a signal from the NH proton, mostly
likely due to exchange with water present in the solvent.
It was thought that the relatively low intensity of the NMR sig-
nals of the 5-methoxy complex could be due to the presence of
small amounts of vanadium(IV) impurities in the sample. To con-
firm this, solid state X-band EPR spectra were collected for all com-
plexes. Fig. 1 shows a representative EPR spectrum (black), along
with the associated fit (red). The EPR spectrum is rhombic
(gx = 1.9797, gy = 1.9806, gz = 1.9461, Ax = Ay = 179 MHz and
Az = 492 MHz[18]) and the values for the fit are well within the
range for a typical oxovanadium(IV) species (gz < gx~gy < 2 and Az >-
Ax~Ay ꢀ0).[19] There is a second, broad background signal cen-
tered around g = 2.00 in all spectra that couldn’t be accounted
for in the simulation parameters. All complexes had a measurable
EPR spectrum, but that of the 5-methoxy complex was the most
intense per mole of material.
3.1.1. Oxidation studies
As has been shown by many authors,[3,23,24] dioxovana-
dium(V) complexes can act as efficient catalysts in the oxida-
tion of styrene. The yields of the reactions are typically
moderate and product distributions can be quite large, depend-
ing on the ligand framework. Styrene oxide (SO) and other oxi-
dation products, including benzaldehyde (BzAld), benzoic acid
(BzAcid) and 1,2-dihydroxyphenylethane (PEOH) are the major
products previously identified in these reactions. In this study,
we looked to see how changing the electron densities within
the ligand framework and therefore the redox potential of the
complex would affect the complexes’ catalytic activity and
whether oxidant structure would affect the product
distributions.
The oxidation of styrene was performed by dissolving styrene
in 25 mL of acetonitrile and heating to 80 °C before adding the
vanadium catalyst and the appropriate oxidant. The reactions
were run overnight with aliquots taken at 30 min, 1, 2, 4, 8 and
24 h to monitor progress. In all cases, the there was little to no
change in the product concentrations or distribution after 2 h
and all values reported are from this time point. A variety of con-
trol reactions were run in order to establish baseline results to
compare our work to and the results summarized in Table 2.
When either the vanadium precursor (VO(acac)2) or the proteo-
catalyst (1b) was added to styrene in a 100:1 ratio with no oxi-
dant present, no reaction was seen to have taken place. Under
conditions where just 30% hydrogen peroxide was used with no
added catalyst, a low conversion of 2% was observed, with ben-
zaldehyde being the major product (71%) and only small amounts
of styrene oxide (29%).
All the oxidation reactions were done in duplicate and the
reported results are the averages seen across the reactions and
the results of the oxidation reactions can be seen in Table 2. The
major products seen in these reactions were styrene oxide, ben-
zaldehyde, and benzoic acid, which were expected based on liter-
ature reports of similar systems.[3] To a lesser extent, other
reoccurring products included acetophenone, phenylacetaldehyde,
and 1-phenethylalcohol (Fig. 3). The conversion of the reactions is
low to moderate with the single addition of the oxidant, with the
highest yield in this reaction being between 25% À 50% for these
systems. There is a slight increase in the yield when 3b is used
as the catalyst, but the choice of oxidant (30% hydrogen peroxide
vs. 70% TBHP) has little effect on the overall conversion. All of
the catalysts successfully oxidized styrene but had varying product
distributions. For this work, low conversions with smaller amounts
of oxidant were ideal so as not to produce a large amount of side
The IR spectra for both new compounds showed the expected
changes upon complexation of the ligand to the dioxovandium
(V) core (Table 1). The C@O and C@N stretches in both complexes
are shifted to lower frequencies upon coordination as compared to
the free ligand. There are two stretches in the 910–830 cmÀ1 which
are assigned to the msym(O = V = O) and mas(O = V = O) stretches in
the VO+2 core, with the symmetric stretch appearing at the lower
frequency.[20] Additionally, the IR spectrum of 1b showed a broad
peak centered at ~ 3400 cmÀ1, suggesting water is present in the
sample.
3

V.P. McCaffrey, O.Q. Conover, M.A. Bernard et al.
Polyhedron 205 (2021) 115268
Fig. 1. X band solid state EPR spectrum of dioxovanadium(V) 5-methoxysalisylaldehyde semicarbazone. The microwave frequency used for data acquisition was 9.6773 GHz.
The red trace is the simulations using S = 1/2, ,gx = 1.9797, gy = 1.9806, gz = 1.9461, Ax = Ay = 179 MHz and Az = 492 MHz. ((Colour online.))
products from the further reaction with hydrogen peroxide or
TBHP.
In order to test the oxidative capacity of the catalyst, the reac-
tion was expanded to include a variety of substituted styrenyl
starting materials and two alkynes. Hydrogen peroxide was chosen
as the oxidant, along with the methoxy substituted catalyst (3b)
for all of these reactions, due to the greater specificity of the pro-
duct distribution found previously in the reactions with styrene.
Again, conversions were kept intentionally low for these screening
reactions in order to limit any side oxidations that might take place
with excess amounts of hydrogen peroxide. In all cases, control
reactions that contained the oxidant and starting material only
were run and in all cases showed far less product (5% or less) when
Fig. 2. Cyclic voltammagram of 1b in 0.1 M tetrabutylammonium hexafluorophos-
compared to the reactions run with the dioxovandium catalyst. The
substrates tested, along with conversions and the identified prod-
ucts of the oxidation reactions, are shown in Table 3.
phate (TBAPF6) DMSO solution with a Pt working electrode, Pt counter electrode,
and a Ag wire pseudo-reference electrode in an Ar-filled glovebox. Cyclic voltam-
metry measurements were conducted with a 100 mV/s scan rate.
Table 2
Conversion and product distributions after 120 min. See Fig. 3 for abbreviations used for the products.
Product distribution
Catalyst
VO(acac)2
Oxidant
–
Conversion
0%
SO
PEOH
PhAc
AceP
BzAld
BzAcid
2b
–
0%
–
30% H2O2
30% H2O2
30% H2O2
30% H2O2
30% H2O2
70% TBHP
70% TBHP
70% TBHP
2%
29%
2%
2%
–
–
Trace
–
–
3%
–
trace
–
–
71%
66%
92%
84%
9%
40%
42%
13%
–
7%
–
6%
–
7%
4%
–
VO(acac)2
18%
31%
39%
50%
36%
25%
38%
23%
3%
–
–
–
2%
2%
trace
–
–
–
1b
2b
3b
1b
2b
3b
5%
91%
52%
54%
87%
–
–
–
4

V.P. McCaffrey, O.Q. Conover, M.A. Bernard et al.
Polyhedron 205 (2021) 115268
Fig. 3. Proposed reaction scheme showing the formation pathways for the oxidation of styrene. SO = Styrene oxide. PEOH = 1,2-dihydroxylphenylethane. BzAld = benzalde-
hyde. BzAcid = Benzoic acid. AceP = Acetophenone. PhAc = Phenylacetaldehyde. nd = not detected.
Table 3
Products of oxidation reactions as a function of starting materials.
cies are the active species in the catalytic oxidation reactions of
alkenes.[1] The small amount of V(IV) contaminants in the catalyst
– probably the oxovanadium-ethanol adduct formed from incom-
plete oxidation during the synthesis of the catalyst – are being con-
verted to V(V) in the first step of the reaction as soon as the
peroxides are added.
In order to investigate the effect that varying the electron den-
sity within the ligand framework had on the conversions and yield
of the reaction, the oxidation of styrene under typical conditions
was studied. When hydrogen peroxide was used as the oxidant
in the presence of the catalysts, the nitro (1b) and proteo (2b) sub-
stituted complexes gave predominantly oxidative cleavage prod-
ucts with benzaldehyde (BzAld) as the major cleavage product
4. Discussion
Two new dioxovanadium(V) complexes with electron with-
drawing (–NO2) and electron donation substituents (–OCH3) on
the ligand framework were successfully synthesized and charac-
terized. Although x-ray structures were not able to be collected,
the structures were confirmed through other spectroscopic tech-
niques. These new complexes were used in conjunction with the
unsubstituted form to evaluate how altering the electronic struc-
ture of the ligand framework would affect the complexes’ catalytic
activity towards the oxidation of styrene. As is noted above, there
are small amounts of contaminants from V(IV) species producing
an EPR signal. However, it has been well established that V(V) spe-
5

V.P. McCaffrey, O.Q. Conover, M.A. Bernard et al.
Polyhedron 205 (2021) 115268
(see Fig. 3 for the full reaction scheme). Benzoic acid (BzAcid) was
seen in most reactions, along with other products including ace-
tophenone (AceP), and phenylacetaldehyde (PhAc). 1,2-Dihydrox-
yphenylethanol (PEOH) was seen as a product only when the
nitro-substituted complex 1b was used at the catalyst, contrary
to previous reports.[3] When the ligand was substituted with the
electron donating methoxy group (3b), the yields increased slightly
and product distribution was inverted. In these reactions, styrene
oxide was the major product, with the only side product in the
reaction being small amounts of oxidative cleavage to form ben-
zaldehyde. It is expected that the yields can be increased if addi-
tional oxidant is added over the course of reaction, and further
work optimizing the yields while maintaining the selectivity of
these reactions is currently underway.
In order to show the efficiency and selectivity of these new cat-
alysts over other systems, the oxidation of styrene using vanadyl
acetylacetonate (VO(acac)2) under identical conditions was per-
formed (entry 4, Table 2). The efficiency of the new catalysts is
superior both in conversion (18% for VO(acac)2 vs an average of
40% conversion the dioxovanadium complexes 1a-1c) and resulted
in narrower product distribution and greater selectivity of prod-
ucts. In general, fewer compounds were seen in the product distri-
bution using complexes 1a-1c and the product distribution tended
to have greater selectivity for either the benzaldehyde (complexes
1a and 1b) or styrene oxide (complex 1c) with less overoxidation
or further reactions.
Both vanadium(IV)[25,26] and vanadium(V)[3,27] have been
shown to be effective oxidants towards styrene and other alkenyl
containing reagents and the reactivity of complexes 1a-1c is com-
parable to others vanadium(IV) precatalysts and vanadium(V) cat-
alysts reported in the literature.[1]. Typically, these reports have
focused on the optimization of the oxidation reaction and conver-
sions range from 35 to 98% for a variety of complexes, with the
higher conversions coming from reactions with relatively high cat-
alyst to styrene ratios.[27] In our work, the conversions were kept
intentionally low to minimize side reaction that could occur with
excess oxidant. In these reports, the product distribution of the
products is quite large with benzaldehyde and benzoic acid being
the major products.[26] In many reports, styrene oxide is the
minor product in the reactions, where in this work we were able
to form the epoxide in a 9:1 or 8:1 ratio through a simple substitu-
tion change on the ligand framework.
driving the differences in the chemistry leading to the different
products in the oxidation reactions.
When the oxidant was changed to TBHP, for the reactions using
1b and 2b as the catalyst, the amount of oxidative cleavage was
dramatically reduced, shifting the major product of all reactions
to styrene oxide. The shift in product distribution can be rational-
ized by considering the mechanisms that govern the different reac-
tions. Epoxidation of alkenes using peroxides and vanadium(V) (or
V(IV)) catalysts is thought to proceed through a three membered
ring active species[28,29] (Scheme 2) which is formed through
direct reaction of oxidant with the vanadium core. This species
then goes on to react with the alkenyl moiety of the styrene to
form the epoxide[30] and it is thought that this is a fast reaction.
If there is base present, the epoxide can ring open with subsequent
coordination to the vanadium center. However, there is a compet-
ing decomposition reaction for the excess peroxide present to form
HOꢁ (or ROꢁ in the case of TBHP) radicals which can then lead to
oxidative cleavage.[1] If hydrogen abstraction occurs while the
organic species is coordinated to the vanadium center through a
CAOAV bond, there is a fast CAC bond breaking reaction[31] that
can then occur which leads to carbonyl products. The greater ten-
dency of the HOꢁ radical as compared to the ROꢁ radical to abstract
H is most likely leading to greater amounts of cleavage products in
these reactions (Table 2).[32]
However, the reactions where 3b is used as the catalyst are rel-
atively insensitive to these cleavage reactions, no matter the oxi-
dant used. This could be due to a decreased bonding affinity of
the vanadium towards the electron rich oxygen or through a mis-
match in orbital energies. The specific mechanism is currently
under investigation.
The reaction scheme shown in Fig. 3 was developed to under-
stand the progression of the oxidation reactions. In the proposed
reaction pathway, the styrene is initially oxidized to the styrene
oxide. At this point, ring opening to form 1,2-dihydrox-
yphenylethanol can occur with hydroxide that is present from
the oxidant (hydrogen peroxide or TBHP). It is thought that it is
this species that undergoes oxidative cleavage to form benzalde-
hyde, which is then oxidized to benzoic acid. Alternatively, an
additional oxidative pathway is the formation of the phenethylal-
cohols with fast oxidation to the carbonyl products acetophenone
or phenylacetaldehyde. However, the phenethylalcohols have not
been detected in the reaction mixtures analyzed in this work.
Originally, it was thought that the differences in the product
distributions and yields could be explained by the redox potential
of the complexes. However, the values measured for the potentials
of the three complexes did not offer any explanation. The redox
potentials for the 3b and 2b (-1.56 V and À1.47 V, respectively)
are much more similar than the potential seen for 1b (-0.96 V).
These values mirror the trends that are seen in the pKa values of
starting salicylaldehyde from which the ligand was derived, sup-
porting the idea that the reductions in the CV are ligand centered
and not metal centered.[17] It is likely that by changing the sub-
stituents on the ligand, there is a reordering of the frontier molec-
ular orbitals where the oxidation chemistry is occurring that is
Scheme 2. Proposed catalytic cycle for the oxidation of styrene.
6

V.P. McCaffrey, O.Q. Conover, M.A. Bernard et al.
Polyhedron 205 (2021) 115268
[5] Z. Bourhani, A.V. Malkov, Ligand-accelerated vanadium-catalysed epoxidation
in water, Chemical Communications 36 (2005) 4592–4594.
[6] J. Mottweiler, M. Puche, C. Räuber, T. Schmidt, P. Concepción, A. Corma, C.
Bolm, Copper- and Vanadium-Catalyzed Oxidative Cleavage of Lignin using
Dioxygen, ChemSusChem 8 (2015) 2106–2113.
[7] M.D. Kärkäs, B.S. Matsuura, T.M. Monos, G. Magallanes, C.R.J. Stephenson,
Transition-metal catalyzed valorization of lignin: the key to a sustainable
carbon-neutral future, Org. Biomol. Chem. 14 (2016) 1853–1914.
[8] S. Son, F.D. Toste, Non-Oxidative Vanadium-Catalyzed C-O Bond Cleavage:
Application to Degradation of Lignin Model Compounds, Angew. Chem., Int. Ed.
49 (22) (2010) 3791–3794.
[9] R. Rinaldi, R. Jastrzebski, M.T. Clough, J. Ralph, M. Kennema, P.C.A. Bruijinincx,
B.M. Weckhuysen, Paving the Way for Lignin Valorisation: Recent Advances in
Bioengineering, Biorefining and Catalysis, Angew. Chem. 55 (2016) 8146–
8215.
[10] P. Noblía, E.J. Baran, L. Otero, P. Draper, H. Cerecetto, M. González, O.E. Piro, E.E.
Castellano, T. Inohara, Y. Adachi, H. Sakurai, D. Gambino, ‘‘New Vanadium(V)
Complexes with Salicylaldehyde Semicarbazone Derivatives: Synthesis,
Characterization, and in vitro Insulin-Mimetic Activity À Crystal Structure of
[VvO2(salicylaldehyde semicarbazone)]” Eur, J. Inorg. Chem. 322–328 (2004).
[11] G. Scalese, J. Benítez, S. Rostán, I. Correia, L. Bradford, M. Vieites, L. Minini, A.
Merlino, E.L. Coitiño, E. Birriel, J. Varela, H. Cerecetto, M. González, J.C. Pessoa,
D. Gambino, Expanding the family of heteroleptic oxidovanadium(IV)
compounds with salicylaldehyde semicarbazones and polypyridyl ligands
showing anti-Trypanosoma cruzi activity, J. Inorg. Biochem. 147 (2015) 116–
125, https://doi.org/10.1016/j.jinorgbio.2015.03.002.
[12] N.A. Lewis, F. Liu, L. Seymour, A. Magnusen, T.R. Erves, J.F. Arca, F.A. Beckford,
R. Venkatraman, A. González-Sarrías, F.R. Fronczek, D.G. VanDerveer, N.P.
Seeram, A. Liu, W.L. Jarrett, A.A. Holder, Synthesis, Characterisation, and
Preliminary In Vitro Studies of Vanadium(IV) Complexes with a Schiff Base and
Thiosemicarbazones as Mixed Ligands, Eur. J. Inorg. Chem. 2012 (4) (2012)
664–677, https://doi.org/10.1002/ejic.v2012.410.1002/ejic.201100898.
[13] G. Licini, V. Conte, A. Coletti, M. Mba, C. Zonta, Recent advances in vanadium
catalyzed oxygen transfer reactions, Coord. Chem. Rev. 255 (19-20) (2011)
2345–2357, https://doi.org/10.1016/j.ccr.2011.05.004.
The methoxy substituted catalyst was chosen in order to test
the generality of the catalyst towards other styrenyl starting mate-
rials (Table 3). Substitution at either the alpha- or the beta-carbon
of the styrene has little effect on the yields of the epoxidation reac-
tion (entries 1 and 2), with similarly low yields of the associated
oxidative cleavage products. Even the relatively sterically hindered
cis-stilbene (entry 2) was able to undergo successful epoxidation
reactions. Substitution on the aromatic ring of the starting material
had an interesting effect on the product distribution. The more
electron-rich vinyl anisole underwent epoxidation at yields com-
parable to styrene (entry 3), while 4-nitrostyrene had no reaction
at all under these conditions (entry 4). Most likely, the nitro substi-
tution lowers the energies of the FMOs of the double bond enough
so that there isn’t an effective bonding interaction available.
5. Conclusions
In summary, we have demonstrated that reactivity of dioxo-
vanadium(V) catalysts towards styrene epoxidation and/or oxida-
tive cleavage can be controlled through changing the
substituents on the ligand framework. We have shown that the
we can tune the reactivity of the catalyst through an easily acces-
sible synthetic framework. We are working to further elucidate the
mechanism of these reactions and increase the yields of these
reactions.
CRediT authorship contribution statement
[14] A. Coletti, P. Galloni, A. Sartorel, V. Conte, B. Floris, Salophen and salen oxo
vanadium complexes as catalysts of sulfides oxidation with H2O2: mechanistic
insights, Catal. Today 192 (1) (2012) 44–55, https://doi.org/10.1016/
j.cattod.2012.03.032.
Vanessa P. McCaffrey: Conceptualization, Supervision, Writing
- review & editing. Olivia Q. Conover: Investigation, Writing - orig-
inal draft. Michael A. Bernard: Investigation, Writing - original
draft. Jonathan T. Yarranton: Investigation. Nicholas R. Lessnau:
Investigation. Jordan P. Hempfling: Investigation.
[15] M.M. Jones, R.A. Rowe, Inorganic Syntheses 5 (1957) 113–116.
[16] P. Noblıa, M. Vieites, B. Parajón-Costa, E. Baran, H. Cerecetto, P. Draper, M.
´
González, O. Piro, E. Castellano, A. Azqueta, A. Ceráin, A. Monge-Vega, D.
Gambino, Vanadium(V) complexes with salicylaldehyde semicarbazone
derivatives bearing in vitro anti-tumor activity toward kidney tumor cells
(TK-10): crystal structure of [VVO2(5-bromosalicylaldehyde semicarbazone)],
J. Inorg. Biochem. 99 (2005) 443–451.
Declaration of Competing Interest
[17] S. Stoll, A. Schweiger, A. EasySpin, a Comprehensive Software Package for
Spectral Simulation and Analysis in EPR, J. Magn. Reson. 178 (2006) 42–
55.
[18] D. Sanna, K. Varnágy, S. Timári, G. Micera, E. Garribba, VO2+ Complexation by
Bioligands Showing KetoEnol Tautomerism: A Potentiometric, Spectroscopic,
and Computational Study, Inorganic Chemistry 50 (2011) 10328–10341.
[19] H. Taguchi, K. Isobe, Y. Nakamura, S. Kawaguchi, ‘‘cis-Dioxovanadium(V) and
Mixed-Valence Divanadium(IV, V) Complexes Containing: b-Diketonate and
Heterocyclic Nitrogen-base Ligands‘‘ Bull, Soc. Soc. Japan 51 (7) (1978) 2030–
2035, https://doi.org/10.1246/bcsj.51.2030.
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgements
The authors would like to thank Prof. James K. McCusker at
Michigan State University and Prof. David W. MacMillan at Prince-
ton for useful conversations regarding the mechanism of these
reactions. The work was supported by a grant from the Hewlett-
Mellon Fund for Faculty Development at Albion College, Albion,
MI and by the Foundation for Undergraduate Research, Scholarship
and Creative Activity (FURSCA) at Albion College. OQC, MAB would
like to acknowledge FURSCA for summer funding to complete this
work. The authors would also like to thank the reviewers for their
careful reading and suggestions on improving this work.
[20] Janusz Szklarzewicz Anna Jurowska Dariusz Matoga Krzysztof Kruczała
_
Grzegorz Kazek Barbara Mordyl Jacek Sapa Monika Papiez 185 2020 114589
10.1016/j.poly.2020.114589
[21] D.S. Silvester, A.J. Wain, L. Aldous, C. Hardacre, R.G. Compton, Electrochemical
reduction of nitrobenzene and 4-nitrophenol in the room temperature ionic
liquid [C4dmim[N(Tf)2, Journal of Electroanalytical Chemistry 596 (2006) 131–
140.
[22] V. Conte, B. Floris, Vanadium catalyzed oxidation with hydrogen peroxide,
Inorg. Chim. Acta 363 (2010) 1935–1946.
[23] P. Noblía, E.J. Baran, L. Otero, P. Draper, H. Cerecetto, M. González, O.E. Piro, E.E.
Castellano, T. Inohara, Y. Adachi, H. Sakurai, D. Gambino, Eur. J. Inorg. Chem.
(2004) 322–328.
[24] V.K. Singh, A. Maurya, N. Kesharwani, P. Kachhap, S. Kumari, A.K. Mahato, V.K.
Mishra, C. Haldar, ‘‘Synthesis, characterization, and catalytic oxidation of
styrene, cyclohexene, allylbenzene, and cis-cyclooctene by recyclable
polymer-grafted Schiff base complexes of vanadium(IV), Journal of
Coordination Chemistry 71 (4) (2018) 520–541, https://doi.org/10.1080/
00958972.2018.1434516.
References:
[1] M. Sutradhar, L.M.D.R.S. Martins, M.F.C. Guedes da Silva, A.J.L. Pombeiro,
Vanadium complexes: Recent progress in oxidation catalysis, Coordination
Chemistry Reviews 301-302 (2015) 200–239.
[2] M. Maurya, A. Kumar, M. Ebel, D. Rehder, Synthesis, Characterization,
Reactivity, and Catalytic Potential of Model Vanadium(IV, V) Complexes with
Benzimidazole-Derived ONN Donor Ligands, Inorg. Chem. 45 (2006) 5924–
5937.
[3] A.V. Malkov, L. Czemerys, D.A. Malyshev, Vanadium-Catalyzed Asymmetric
Epoxidation of Allylic Alcohols in Water, Journal of Organic Chemistry 74 (9)
(2009) 3350–3355, https://doi.org/10.1021/jo900294h.
[4] W. Zhang, H. Yamamoto, Vanadium-Catalyzed Asymmetric Epoxidation of
Homoallylic Alcohols, Journal of the American Chemical Society 129 129 (2)
[25] F. Marchetti, C. Pettinari, C. Di Nicola, R. Pettinari, A. Crispini, M. Crucianelli, &
A. Di Giuseppe
[26] M.R. Maurya, B. Uprety, N. Chaudhary, F. Avecilla, ‘‘Synthesis and
Characterization of Di-l-Oxidovanadium(V), Oxidoperoxido-Vanadium(V)
and Polymer Supported Dioxidovanadium-(V) Complexes and Catalytic
Oxidation of Isoeugenol, Inorg. Chim. Acta 434 (2015) 230–238.
[27] M.R. Maurya, C. Haldar, A. Dumar, M.L. Kuznetsov, F. Avecilla, J.-C. Pessoa,
Vanadium complexes having [VO]2+, [VO]3+ and [VO+2 cores with hydrazones of
2,6-diformyl-4-methylphenol: synthesis, characterization, reactivity, and
catalytic potential, Dalton Transactions 42 (2013) 11941–11962.
[28] K.A. Jorgenson, Transition-metal-catalyzed epoxidations, Chem. Rev. 89 (1989)
431–458.
(2007)
286–287,
https://doi.org/10.1021/ja067495y10.1021/ja067495y.
s00110.1021/ja067495y.s002.
7

V.P. McCaffrey, O.Q. Conover, M.A. Bernard et al.
Polyhedron 205 (2021) 115268
[29] B.J. Hamstra, G.J. Colpas, V.L., Pecaro ‘‘Reactivity of Dioxovanadium(V)
Complexes with Hydrogen Peroxide: Implications for Vanadium
Holoperoxidase”, Inorganic Chemistry 37 (1998) 949–955, https://doi.org/
10.1021/ic9711776.
[30] H. Liu, H. Li, N. Luo, F. Wang, Visible-Light-Induced Oxidative Lignin C-C Bond
Cleavage to Aldehydes Using Vanadium Catalysts, ACS Catalysis 10 (1) (2020)
632–643, https://doi.org/10.1021/acscatal.9b0376810.1021/acscatal.9b03768.
s001.
[31] B.P. Roberts, A.J. Steel, An Extended Form of the Evans-Polanyi Equation: a
Simple Empirical Relationship for the Prediction of Activation Energies of
Hydrogen-atom Transfer Reactions, J. Chem. Soc. Perkins Trans. 2 (10) (1994)
2155–2162.
8