Robust and electron deficient oxidovanadium(iv) porphyrin catalysts for selective epoxidation and oxidative bromination reactions in aqueous media

Article abstract of DOI:10.1039/c8gc03909g

meso-Tetrakis(3,5-dimethoxyphenyl)porphyrinatooxidovanadium (1) and β-octabromo-meso-tetrakis(2,6-dibromo-3,5-dimethoxyphenyl)porphyrinatooxidovanadium (2) were synthesized in excellent yields. The synthesized oxidovanadium(iv) porphyrin complexes were characterized by various spectroscopic methods such as UV-Vis, FT-IR, EPR and MALDI-TOF mass spectrometry, as well as by single crystal X-ray crystallography. The acetonitrile coordinated porphyrin 1 is relatively planar whereas 2 is highly nonplanar as shown by their crystal structures and also by electrochemical studies. Selective epoxidation studies of alkenes were successfully carried out in CH₃CN/H₂O as the solvent mixture at ambient temperature resulting in very high TOF numbers (12227-14347 h^-1 for 2) even with low catalyst loadings. Remarkably, 2 biomimics the vanadium bromoperoxidase (VBrPO) enzyme with extremely high TOF values (83333-87719 h^-1) for the oxidative bromination of thymol and some other phenols. Both the catalysts were successfully recovered at the end of the reactions, indicating their viability and industrial applicability.

Full text of DOI:10.1039/c8gc03909g

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Robust and Electron Deficient Oxidovanadium(IV)
Porphyrin Catalysts for Selective Epoxidation and
Oxidative Bromination Reactions in Aqueous Media
Tawseef Ahmad Dar,† Bhawna Uprety†§, Muniappan Sankar*† and Mannar R. Maurya*†
†Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee – 247667, India.
KEYWORDS: Oxidovanadium(IV) porphyrins, Catalysis, Epoxidation, Oxidative bromination,
Aqueous media
ABSTRACT: Meso-tetrakis(3,5-dimethoxyphenyl)porphyrinatooxidovanadium (1) and β-
octabromo-meso-tetrakis(2,6-dibromo-3,5-dimethoxyphenyl)porphyrinatooxidovanadium
(2)
were synthesized in excellent yields. The synthesized oxidovanadium(IV) porphyrin complexes
were characterized by various spectroscopic methods such as UV-Vis, FT-IR, EPR and MALDI-
TOF mass spectrometry as well as by single crystal X-ray crystallography. The acetonitrile
coordinated porphyrin 1 is relatively planar whereas 2 is highly nonplanar as observed by their
crystal structures and also by electrochemical studies. Selective epoxidation studies of alkenes
were successfully carried out in CH3CN/H2O solvent mixture at ambient temperature resulting in
very high TOF numbers (12227-14347 h-1 for 2) even with low catalyst loading. Remarkably, 2
biomimics vanadium bromoperoxidase (VBrPO) enzyme with extremely high TOF value
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(83333-87719 h-1) for oxidative bromination of thymol and some other phenols. Both the
catalysts were successfully recovered at the end of the reactions indicating their viability and
industrial applicability.
INTRODUCTION
Epoxides are very useful precursors as they can be converted into a wide variety of laboratorial
and industrially important products like surfactants, polyurethanes, resins, glycols etc.1–3 For
obtaining epoxides, the main protocol is epoxidation of alkenes with or without the help of
catalysts.4 A wide variety of transition metal based catalysts speed up an equally diverse number
and type of chemical reactions such as polymerization, oxidation, hydroxylation, coupling and so
on.5,6 Porphyrin is an important ligand that forms stable complexes with a variety of metal ions
to generate metalloporphyrins.7–10 Metalloporphyrins have good thermal and chemical stability
in addition to their strong absorption in the visible region of the spectrum as also being versatile
for further modifications in terms of conformational and redox properties.11–13 On account of
their customized properties, metalloporphyrins find useful applications in the fields of DSSCs
(dye sensitized solar cells)10,14 sensing15 and NLO (Non-linear optics).16
A number of iron, molybdenum and manganese based metalloporphyrins have been used as
homogenous catalysts for oxygenation reactions.17–19 These catalysts, however, suffer from some
snags like; (a) the macrocyclic ring is quite susceptible towards oxidative cleavage; (b)
aggregation may occur due to π-π interactions; (c) lower rate of conversion i.e TOF; (d) poor
selectivity in terms of products formed during the reaction. However, some metalloporphyrin
based heterogeneous catalysts have been reported lately to overcome some of these drawbacks
with good efficiency.20,21 Recently, Oxidovanadium(IV) porphyrins have gained significant
ground as useful moieties with wide applications in the fields of therapeutics and HIV22, 3D-
2

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supramolecular assemblies23 and catalysis.24,25 VOTPP and its immediate derivatives have been
successfully utilized for epoxidation of various alkenes under homogenous and heterogenous
conditions.24,26 However, low turnover number remains a major drawback of these catalysts
which has therefore become the focus of researchers globally. In addition to molecular oxygen,
H2O2 is a widely used, environmental friendly, biologically relevant and an easy to handle
oxidant and in coalition with NaHCO3 as promoter gives good conversion with excellent
selectivity for a variety of olefins.27–30 The interaction between NaHCO3 and H2O2 stabilizes the
oxidoperoxido intermediates and results in the formation of peroxymonocarbonate ions, which
are much stronger nucleophiles as compared to H2O2, thereby increasing the efficiency of the
metal catalysts being used by many folds.31,32
Terpenes are a useful class of natural products that lead to commercially profitable species
which are medicinally important in being anti-microbial, anti-cancer, anti-inflammatory etc.33
Oil of thyme extracted from thyme (Thymus vulgaris) contains 20-54% thymol which has strong
antiseptic properties and is used in mouth washes.34,35 Oxidative bromination is an indispensable
reaction because the carbon bromine bond renders the organic compounds more lipophilic and
hence biologically more penetrating.36 Traditionally, bromination has been carried out by using
reagents like liquid bromine,37 hypobromites,38 NBS (N-Bromosuccinimide)39 and so on.
Vanadium dependent haloperoxidases occur naturally in some marine algae, fungi and lichens,
and the active center in these vanadate-dependent haloperoxidases is a hydrogenvanadate(V)
coordinated to an imidazole nitrogen atom of the histidine subunit.40 These enzymes catalyze the
electrophilic halogenation of organic substrates in the presence of peroxides and the reaction
proceeds via oxidation of the halide in question (vide infra). The active species in the catalytic
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reaction is an oxidoperoxidovanadium(V) intermediate, which is a much stronger oxidizing agent
than H2O2.
H2O2 + X- + H+
H2O + HOX (X = Cl, Br, I)
A plethora of structural and/or functional models of vanadium haloperoxidases with
oxidovanadium(IV/V) and dioxidovanadium(V) centers coordinated to N, O, S donor ligands are
commonly encountered in literature.41–43 We have recently reported the catalytic efficacy of an
oxidovanadium(IV) porphyrin towards epoxidation of cyclohexene and related alkenes, in the
presence of H2O2 as the terminal oxidant and NaHCO3 as the catalyst promoter.44 In the current
study, we have further extended our efforts towards the application of substituted porphyrins as
potential industrial catalysts with huge TOF (h-1) numbers. Herein, we report two substituted
porphyrins (1 and 2) bearing diverse electronic environments (Figure 1), with direct
consequences on reactivity and catalytic activity. We have used VOTPP(OMe)8 (1) and
VOTPP(OMe)8(Br)16 (2) as catalyst precursors for the oxidation of olefins, considering styrene
as the ideal substrate; as well as oxidative bromination of a monoterpenoid, thymol and some
other phenols, in presence of H2O2 as the oxidant. We have encountered unprecedented TOF (h-
1) numbers and shorter reaction times as compared to literature reports, indicating higher
efficiency of the present catalyst systems. Additionally, the catalysts were successfully recovered
at the end of the reactions indicating their stability as well as recyclability for the catalytic
reactions.
Results and Discussion:
Oxidovanadium(IV) porphyrins VOTPP(OMe)8 (1) and VOTPP(OMe)8(Br)16 (2) (Figure 1)
were synthesized using modified literature methods. Free-base 5,10,15,20-tetrakis(3,5-
dimethoxyphenyl)porphyrin, H2TPP(OMe)8, was synthesized using pyrrole and 3,5-
4

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dimethoxybenzaldehyde using Adler-Longo method.45 VOTPP(OMe)8 (1) was prepared using
VOSO4 and DMF under argon atmosphere in 96% yield.23,44 VOTPP(OMe)8(Br)16 (2) was
synthesized from VOTPP(OMe)8 (1) using liq. Br2 as the brominating agent in 83% yield.46 1
and 2 were characterized by UV-Visible, FT-IR and EPR spectroscopic techniques, MALDI-
TOF mass spectrometry (Figures S1-S5 in ESI) and single crystal XRD analysis.
R
X
X
X
X
X
X
O
V
N
N
N
N
R
R
X
X
R
Br OMe
,
OMe
X= H
;
X= Br
R =
,
R =
OMe
1
Br OMe
2
Figure 1. Molecular structures of porphyrins 1 and 2 used for catalysis.
Figure 2 represents the UV-Visible spectra of porphyrins 1 and 2 in CH2Cl2 at 298 K. 1 gave a
typical absorption spectrum characteristic of oxidovanadium(IV) porphyrins - one Soret and one
Q-band with the Soret band being red shifted (5 nm) as compared to 3,5-
dimethoxyphenylporphyrin (H2TPP(OMe)8). Porphyrin 2 exhibited an absorption spectrum
which is fairly red-shifted as compared to 1 in terms of the Soret (Δλmax = 38 nm) and Q bands
as shown in Figure 2 and Table S1 in the supporting information (ESI). The IR spectrum of 1
shows a characteristic peak at ν ~1005 cm-1 due to (V=O) stretching; the coordination was
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further confirmed by the absence of N-H stretching frequency which is otherwise observed for
free base porphyrin (H2(TPP)(OMe)8) at ν ~3311 cm-1. Similarly, the IR spectrum of 2 exhibits
stretching frequencies for C-Br bond as well as V=O bond at ν ~682 cm-1 and ν ~1031 cm-1,
respectively.
Figure 2. UV-Visible spectra of 1 and 2 in CH2Cl2 at 298 K.
Electrochemical Studies:
Figure 3. Cyclic voltammograms of 1 and 2 in CH2Cl2 at 298 K with reference to Ag/AgCl
electrode.
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Cyclic voltammetric studies of 1 and 2 were carried out in distilled CH2Cl2 at 298 K using
Ag/AgCl reference electrode, Pt working electrode, Pt wire counter electrode and recrystallized
TBAPF6 as the supporting electrolyte (Figure 3). The first oxidation potential of 2 is 340 mV
anodically shifted as compared to 1. Similarly, the first reduction potential of 2 is also 320 mV
anodically shifted as compared to 1. These results reveal the electron deficient nature of the
porphyrin macrocycle (2) due to electron withdrawing hexadecabromo substituents. The first
oxidation potential of 2 is 460 mV anodically shifted as compared to H2TPP(OMe)8. Likewise,
the first and second reduction potentials of 2 are anodically shifted by 360 mV and 340 mV,
respectively as compared to H2TPP(OMe)8. These results are quite expected owing to the
nonplanarity and presence of more electronegative bromo groups in 2 (Figure 3).
Thermogravimetric Studies:
The thermogravimetric profiles of 1 and 2 using alumina powder as reference were recorded
o
o
from RT to 1000 C with a heating rate of 10 C/min. The resulting thermograms are given in
Figures S7 and S8 in the ESI. Both 1 and 2 displayed good thermal stability with 1 being stable
upto 336 °C and 2 being stable up to 273 °C. The lower stability of 2 could be explained on the
fact that the C–Br bond (285 kJ/mol) is weaker and more labile as compared to C–O bond (358
kJ/mol). Thermogravimetric profile of H2TPP(OMe)8 is also given in Figure S6 in the ESI.
EPR Spectral Studies:
X-band EPR spectra of 1 and 2 were recorded in distilled toluene. Both 1 and 2 exhibited typical
EPR spectra with neatly resolved 51V hyperfine lines at 120 K (Figure 4). The number of lines in
the axial EPR spectra of both 1 and 2 were equal to eight corresponding to the I value of
vanadium nucleus (I = 7/2).
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Figure 4. X-band EPR spectra of 1 and 2 in toluene at 120 K. EPR parameters: microwave
frequency, 9.453 GHz; incident microwave power, 0.189 Mw.
From the anisotropic EPR spectra, gǁ and g⟘ pe values were obtained as 1.965 and 1.999
respectively for 1. For 2 the corresponding values were observed as 1.966 and 2.000
respectively. Similarly, Aǁ and A⟘ values were obtained as 160.67 × 10-4 cm-1 and 57.20 × 10-4
cm-1 respectively for 1. The corresponding values were observed as 158.95 × 10-4 cm-1 and 58.15
× 10-4 cm-1 respectively for 2. All these values are within the permissible range for axially
compressed d1 systems.
DFT Studies and Geometry Optimization
DFT studies to optimize geometries were carried out using B3LYP functional with LANL2DZ
basis set. Figures 5 and S9 in the ESI represent the top and side views for 1 and 2 in their
optimized geometries in the gaseous phase. 1 is relatively planar with Δ24 = 0.0311 Å and ΔCβ =
± 0.0493Å from the porphyrin mean plane. However, vanadium is clearly located above the
plane by a distance of 0.526 Å. The bond distances V=O and Cβ−Cβ were found to be 1.595 Å
and 1.372 Å, respectively in case of 1. On the other hand, 2 is clearly nonplanar having a saddle
shaped conformation with Δ24 = 0.548 Å and ΔCβ = ± 1.1135 Å from the porphyrin mean plane.
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Vanadium metal is located above the plane by about 0.545 Å. The bond distances V=O and
Cβ−Cβ in case of 2 were found to be 1.593 Å and 1.378 Å respectively.
(a)
(b)
Figure 5. B3LYP/LANL2DZ optimized geometry of 2 showing (a) top view and (b) side view.
In the side view, front side β-bromo and meso-phenyl substituents are not shown for clarity.
Single Crystal XRD Analysis
Diffraction quality single crystals for both 1 and 2 were obtained by slow diffusion of hexane
into their saturated acetonitrile solutions at room temperature. The ORTEPs of 1 and 2 are shown
in Figure 6 and the corresponding crystallographic data are listed in Table S2 in the supporting
information. The selected average bond lengths and bond angles are listed in Table S3 in the ESI.
The XRD studies confirmed that 1 is relatively planar while 2 is highly distorted (saddle
conformation) from the mean plane with the values of Δ24 being 0.030 Å and 0.495 Å in case of
1 and 2, respectively. β-Pyrrole carbon atoms are also highly deviated from the mean plane in
case of 2 with the corresponding ΔCβ values found to be 0.036 Å and 1.011 Å in case of 1 and 2,
respectively. Similarly, the meso-carbon atoms in 2 are more distorted (ΔCm = 0.044 Å) than
meso-carbon atoms (ΔCm = 0.021 Å) in 1. Likewise, the α-pyrrolic carbon atoms in 1 (ΔCα =
0.026 Å) are less deviated from the mean plane as compared to α-pyrrolic carbon atoms in 2
(ΔCα = 0.396 Å). The highly nonplanar nature of 2 makes the macrocycle robust and the
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hexadeca bromo groups make the porphyrin ring electron efficient. We expect that these two
aspects potentially help porphyrin 2 to perform better in oxidative catalysis as compared 1 and
hence we carried out catalytic studies.
(a) 1 top view
(b) 1 side view
(c) 2 Top view
(d) 2 side view
Figure 6. ORTEPs showing (a) top view of 1 (b) side view of 1 (c) top view of 2 and (d) side
view of 2. The oxidovanadium(IV) center in case of 1 is coordinated to CH3CN. Color coding; N
(light blue), O (red), C (grey black), H (grey white), Br (brown) and V (light pink).
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CATALYTIC STUDIES
Epoxidation of Olefins
The catalytic efficiency of 1 towards olefin epoxidation was explored by conducting a series of
experiments. The catalytic oxidation of styrene by 1 in presence of H2O2 as the oxidant and
NaHCO3 as the promoter gave only one product, i.e., styrene epoxide exclusively (Scheme 1).
O
Catalyst, 30% H2O2
NaHCO3, MeCN, 50 C
Scheme 1. Catalytic epoxidation of styrene.
All the reactions were performed in a 50 mL two-neck round bottomed flask fitted with a
reflux condenser over an oil bath. The reaction conditions were optimized by taking styrene as
the ideal substrate and 1 as the representative catalyst. The reaction products were quantified
using GC/GC-MS analysis. The complete details regarding the epoxidation of styrene under
different reaction conditions are given in the experimental section and are summarized in table 1.
The most ideal/optimized condition for epoxidation of 5 mmol of styrene in 1 h of reaction
time are: catalyst 1 (0.0015 g, 1.63 μmol), 30% aqueous H2O2 (1.69 g, 15 mmol), NaHCO3
(0.126 g, 1.5 mmol), CH3CN:H2O (3:2, 5 mL) and temperature 50 °C. The reaction showed
excellent selectivity with the formation of styrene epoxide as the exclusive product.
To ascertain the role of the catalyst and the promoter in the epoxidation of styrene, we
performed some control experiments. In the first control, the reaction was performed under the
optimized conditions, but in the absence of catalyst, and only 21% conversion was observed.
Next, the reaction under optimized conditions, but in the absence of both catalyst and promoter
gave only 9% conversion.
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Table 1. Oxidation of styrene (0.52 g, 5 mmol) using porphyrin 1 as the catalyst in 1 hour time
scale under different reaction conditions.
Entry
No.
Catalyst
(mg,
30%
NaHCO3
MeCN/H2O
(v/v), ml
Temp.
%
Selectivity
H2O2 (g, (g, mmol)
mmol)
(˚C)
Conversion
µmol)
1
1.5, 1.63 1.69, 15
1.5, 1.63 1.69, 15
1.5, 1.63 1.69, 15
1.5, 1.63 1.69, 15
1.0, 1.10 1.69, 15
2.0, 2.20 1.69, 15
1.5, 1.63 0.565, 5
1.5, 1.63 1.13, 10
1.5, 1.63 1.69, 15
0.168, 2
0.168, 2
0.168, 2
0.168, 2
0.168,2
0.168, 2
0.168, 2
0.168, 2
0.840, 1
5:0
2:1
3:2
4:3
3:2
3:2
3:2
3:2
3:2
3:2
3:2
3:2
60
60
60
60
60
60
60
60
60
60
50
40
77
81
90
88
74
91
15
78
77
89
87
34
100
100
100
100
100
100
100
100
100
100
100
100
2
3
4
5
6
7
8
9
10
11
12
1.5, 1.63 1.69, 15 0.126, 1.5
1.5, 1.63 1.69, 15 0.126, 1.5
1.5, 1.63 1.69, 15 0.126, 1.5
The efficiency of the catalyst was further supported by performing the reaction using
[VO(acac)2] as the catalyst instead of 1 under the optimized conditions; which gave a total
conversion of 37% only. Finally, the optimized reaction without the promoter gave 61%
conversion. All these results indicate that while the catalyst can satisfactorily catalyze the
epoxidation of alkenes, the overall conversion is significantly improved in presence of the
promoter.
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We also examined the catalytic potential of VOTPP(OMe)8(Br)16 (2) towards olefin
epoxidation under the conditions optimized for VOTPP(OMe)8 (1). The bromo derivative (2)
was found to be much more active than the corresponding octamethoxy oxidovanadium(IV)
porphyrin (1) in the oxidation of alkenes, probably due to the difference in electronic
environments around vanadium in the two molecules. Thus, we decided to evaluate the catalytic
efficiency of (2) under the reaction conditions as optimized above for (1), but at lower catalyst
loading and lesser reaction time. Indeed, we observed almost comparable conversions by using
only half the catalyst amount (ca.0.7 μmol, 0.0016 g) in merely 0.5 h of reaction time under the
conditions optimized above for (1). However, further reducing the catalyst amount to 0.4 μmol,
rendered 81% conversion in 0.5 h of reaction time for 5 mmol styrene under the conditions
optimized for; and thus, a minimum of 0.7 μmol of was considered to be ideal for catalytic
activity under the conditions as optimized above.
The higher TOF values observed in the case of VOTPP(OMe)8(Br)16 (2) as compared to
VOTPP(OMe)8 (1) can be seen as a direct consequence of the underlying mechanism for
vanadate catalyzed oxidations. Most vanadium catalyzed oxidations are inspired from mimicking
the active site of vanadium haloperoxidases (where the activated form of the active site is formed
by vanadium metal coordinated to two oxygen atoms, two histidine units and one peroxo unit47
as compared to four pyrollic nitrogens, one oxygen atom and one peroxo unit in the porphyrins 1
and 2). Vanadium haloperoxidases catalyze the oxidation of halides by hydrogen peroxide
thereby achieving the halogenation of organic substrates.48,49 Thus, most vanadium catalyzed
oxidations are known to proceed in the presence of peroxides, and the fundamental step of the
catalytic cycle involves the formation of an oxido-peroxidovanadium intermediate which is a
much stronger oxidant than H2O2.50,51 The oxido-peroxido vanadium species functions as an
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electrophilic electron transfer agent, while oxidizing the organic substrate;52 this implies that an
electron deficient vanadium center would facilitate the formation of the oxido-peroxido
intermediate, thus speeding up the reaction. The two derivatives reported in the present study
differ greatly in terms of electron density at the oxidovanadium(IV) center. Accordingly, the
higher electrophilic nature of vanadium in oxidovanadium(IV) porphyrin 2 can account for its
higher reactivity. The relevant data is presented is Table 2.
The epoxidation of olefins catalyzed by bicarbonate activated peroxide was initially
investigated by Drago and co-workers in their study of the oxidation of chemical warfare agents
and simulant sulphides in mixed tert-butyl alcohol/water solvents53 and was further extended by
Richardson et al.54 who performed a detailed kinetic analysis for bicarbonate mediated
oxidations in water/alcohol mixtures. It was suggested that the key aspect of such systems was
the equilibrium between the bicarbonate and the peroxide to give a peroxymonocarbonate
intermediate (eq. 1). Such equilibrium could be established in minutes, but the reactions
catalyzed only by such systems in the absence of a catalyst required very long reaction times of
upto 1-2 days. Fast and efficient epoxidation of olefins catalyzed synergistically by a metal
catalyst and bicarbonate activated peroxide has been reported successfully by Burgess et al.
using a Mn catalyst.30
-
-
HCO3 + H2O2
HCO4 + H2O (eq. 1)
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Table 2. Oxidation of different olefins using 1.6 μmol of VOTPP(OMe)8 (1) and 0.7 μmol of
VOTPP(OMe)8(Br)16 (2), 30% H2O2 as terminal oxidant and sodium bicarbonate as promoter
using CH3CN/H2O as the solvent mixture under the optimized conditions for 1 h of reaction time
for 1 and 0.5 h of reaction time for 2.
Conv. %
TOF (h-1)
1 2
S. No. Substrate
Product
1
2
O
1
87
95
2719
13768
2
97
82
99
96
3031
2563
14347
13974
O
O
3
O
O
O
4
5
83
94
84
95
2594
2938
12227
13768
O
O
O
6
92
97
2875
14058
7
8
93
81
95
86
2906
2531
13768
12463
O
Therefore, the organic solvent serves to dissolve the product, pushing the equilibrium towards
right, hence “speeding up” the reaction. The mixture of water and an organic solvent additionally
allows the homogenization of the reaction mixture, thus facilitating fast reaction kinetics.
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Oxidative Bromination of Thymol
Several model vanadium compounds as structural and/or functional mimics of vanadium
haloperoxidases (VHPOs) are commonly reported in literature.55-58 Many of these compounds
catalyze various oxidations in the presence of peroxide.59,60 We have performed the VHPOs
activity in aqueous medium using thymol as the ideal substrate and 2 as the catalyst precursor.
The oxidative bromination of thymol carried out in aqueous medium gave three different
products, namely, 2-bromothymol, 4-bromothymol and 2,4-dibromothymol (Table 3). The
products formed were the same as in previous studies reported by some of us61,62 and by other
authors63 (Scheme 2).
OH
OH
OH
OH
Br
Br
H2O2, KBr, HClO4
Catalyst
Br
Br
p-MBrP
o-MBrP
DBrP
Scheme 2. Products obtained in the oxidative bromination of thymol carried out in aqueous
medium using 2 as the catalyst precursor.
For the chosen complex 2 being used as the catalyst precursor, the following parameters were
varied to optimize the reaction conditions for 10 mmol (1.500 g) of thymol; (i) amount of
catalyst (0.00025 g, 0.00050 g and 0.001 g) (ii) amount of HClO4 (10, 20 and 30 mmol) (iii)
amount of KBr (10, 20 and 30 mmol), and (iv) amount of oxidant, H2O2 (10, 20 and 30 mmol).
Table 3 presents the details of conversion under each specified condition. The best suited
conditions for the maximum conversion of 10 mmol of thymol (1.500 g) were found to be:
catalyst (0.00025 g, 0.114 µmol), KBr (2.38 g, 20 mmol), aqueous 30% H2O2 (2.267 g, 20
mmol), HClO4 (2.87 g, 20 mmol, added in four equal portions of 0.717 g) and 20 mL water
(Table 3, entry 4). TOF in this case was found to be a staggering 83333 h-1 with 95% conversion
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rate which is the highest TOF number known for haloperoxidase activity/mimics. Under similar
reaction conditions, catalyst 1 (0.00025 g, 0.272 µmol) was much less efficient and gave a
conversion of 98% and TOF of 36029 h-1 respectively with thymol.
Catalyst 2 was additionally used for oxidative bromination of 10 mmol each of phenol, para-
cresol and salicylaldehyde under the conditions optimized for thymol above. Remarkable
conversion rates and TOFs (h-1) were obtained with all the above substrates. With phenol,
catalyst 2 exhibited 100% conversion rate and a TOF of 87719 h-1, with salicylaldehyde a
conversion rate of 95% and TOF of 83333 h-1 was achieved while para-cresol gave a TOF of
87719 h-1 and a conversion rate of 100% under similar conditions. The details are given in table 4
Table 3. Results for oxidative bromination of thymol (1.50 g, 10.0 mmol), using 2 as the catalyst
precursor, in 20 mL water.
Entry Catalyst (mg, 30 % H2O2
KBr
HClO4
%
Time
(h)
TOF
(h-1)
Selectivity
No.
µmol)
(g, mmol)
(g, mmol)
(g, mmol)
Conv.
o-MBrP
10
12
5
p-MBrP
DBrP
44
31
46
14
6
1
2
0.25, 0.114 2.27, 20 2.38, 20 2.87, 20
0.50, 0.229 2.27, 20 2.38, 20 2.87, 20
97
98
100
95
71
96
56
95
61
98
2
2
2
1
1
1
1
1
1
1
42544
21396
10917
83333
62280
84210
49123
83333
53508
85964
46
57
49
74
81
20
82
45
56
33
3
1.0, 0.458
2.27, 20 2.38, 20 2.87, 20
4*
5
0.25, 0.114 2.27, 20 2.38, 20 2.87, 20
0.25, 0.114 2.27, 20 1.19, 10 2.87, 20
0.25, 0.114 2.27, 20 3.57, 30 2.87, 20
0.25, 0.114 1.13, 10 2.38, 20 2.87, 20
0.25, 0.114 3.39, 30 2.38, 20 2.87, 20
0.25, 0.114 2.27, 20 2.38, 20 1.43, 10
0.25, 0.114 2.27, 20 2.38, 20 4.29, 30
12
13
4
6
76
8
7
10
9
8
46
36
57
9
8
10
10
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Table 4. Results for oxidative bromination of phenolic derivatives (10.0 mmol), using 2 (0.25
mg, 0.114 µmol) as the catalyst precursor, in 20 mL water .
Substrate
Conversion (%)
TOF (h-1)
Products
% Yield
Phenol
100
87719
p-Bromophenol
o-bromophenol
68
32
86
Salicylaldehyde
95
83333
87719
Bromosalicylaldehyde
dibromosalicylaldehyde
Bromo-p-cresol
14
91
9
p-Cresol
100
dibromo-p-cresol
Catalytic Mechanism
The reaction between the catalyst and H2O2 was monitored by UV-Visible spectral titrations,
however, no decisive spectral changes were observed. Thus, in order to ascertain the exact
mechanism underlying our study of epoxidation reactions, MALDI-TOF mass analysis was
performed. 1 and 2 (0.02 mol %) were heated with NaHCO3 (0.4 eq) and H2O2 (2 eq.) in CH3CN
at 60 ˚C for 30 minutes. The reaction mixtures were cooled to room temperature and MALDI-
TOF analysis was performed using HABA matrix. Clear peaks were observed at m/z = 951.18
and m/z = 2213.53 in case of 1 and 2, respectively substantiating the formation of respective
oxidoperoxidovanadium(V) species in solution for both the catalysts as shown in scheme 3 and
Figures S4 and S5 in the ESI.
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-
Na+ + HCO3
NaHCO3
-
-
HCO3 + H2O2
+ H2O
O
HCO4
-
O
O
V
R
-
HCO4
V
O
V
O.S = (IV)
O
O
R
O.S = (V)
O.S = (IV)
Scheme 3. Catalytic mechanism for conversion of alkenes into corresponding epoxides. O.S
refers to oxidation state of the vanadium metal.
The higher catalytic efficiency and the resulting higher TOF (h-1) numbers for 2 as compared
to 1 are a direct consequence of the electron withdrawing bromo substituents on 2 and the
nonplanarity making the oxidovanadium(IV) center in 2 more electrophilic and thus assisting in
the formation of the active oxidoperoxido intermediate. The TOF (h-1) numbers (for epoxidation)
were in the range of 2531 to 3031 h-1 for 1 and 12227 to 14347 h-1 for 2 (five folds higher than
1). The UV-Visible spectra of 1 and 2 after being recovered from the catalytic reaction mixtures
after the first catalytic cycle matched those of the pure samples depicting their good chemical
and thermal stabilities (Figure S10, ESI). The efficiency of the recovered catalysts (1 and 2) was
examined for the epoxidation of cyclohexene under conditions similar to those used for pure
catalysts. Recycled catalyst 1 exhibited a conversion rate of 87% and TOF of 2719 h-1 with
cyclohexene. Similarly, the recycled catalyst 2 exhibited a conversion rate of 90% and TOF of
13043 h-1 with cyclohexene. The small loss in catalytic efficiency can be explained on the basis
that the recovery processes, especially in homogenous catalysis, are not 100% efficient which
inevitably leads to loss of some catalyst amount, more so with low catalyst loading.
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Literature Comparison
TOF (h-1) values in case of epoxidation reaction for 1 and 2 were compared with those reported
in literature (Table 5). It can be seen that, while 1 exhibits TOF values comparable with literature
reports, 2 is found to be much more efficient (5-6 folds higher) and thus is a very potent catalyst
for selective epoxidation of alkenes under ambient conditions. Some catalysts have achieved
higher TOF values with certain alkenes but their TOF values with general alkenes has been very
low (entries 10-13, Table 5). Thus, we can conclude that 2 is a much better catalyst for
selectively epoxidizing alkenes irrespective of the nature of the alkene.
Similarly, the TOF (h-1) values for oxidative bromination of thymol with 2 were also compared
to literature reported values (Table 6) and it can be seen that 2 is by far the most efficient catalyst
for oxidative bromination of thymol. On the basis of its efficiency and reversibility, 2 can be said
to act as a biomimetic catalyst for vanadium bromoperoxidase enzymes found in living species.
Table 5. TOF (h-1) values for epoxidation/oxidation of various alkenes using different catalysts
known in literature.
S.No. Catalyst
Reaction Minimum Maximum ref.a
Time (h) TOF (h-1)
TOF (h-1)
1
2
Ruthenium meso-tetrakis(2,6-
dichlorophenyl)porphyrin.
18-30
1410
4550
21
[Meso-tetrakis(4-trimethylammoniophenyl)
porphyrinato]oxidovanadium(IV)
tetrachloride.
24
24
14.8
5
95.6
26
28
3
4
Polyoxidometalates
4630
Polymer grafted dioxidomolybdenum(VI)
complex
3-4
0.5
1-5
24
52
6566
0.1
664
9560
37.3
38
32
44
64
65
66
5
6
7
8
β-Octachloro oxidovanadium(IV) porphyrin
Titanium(IV) modified silicates
Polyoxidovanadometalate
---
Diiron substituted silicotungstate
385
3.5b
26b
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9
Oxido–peroxido molybdenum(VI) complex
with tridentate Schiff base ligand
1.25
214b
380
129
352
323
240b
67
68
69
70
71
10
11
12
13
Oxidoperoxido tungsten(VI) Complexes
with Secondary hydroxamic Acids
0.25-3
0.1-15
0.1-2
43200
57600
59400
44550
Oxidodiperoxomolybdenum(VI) complex
with salicylaldoxime
Oxido-diperoxotungstate(VI) complex with
8-Quinolinol
Oxido- and Oxoperoxo-molybdenum(VI)
Complexes with Aryl hydroxamates
0.25-3
14
Porphyrin–polyoxidometalate hybrid
2.5-5
9
25
72
15
Oxidovanadium(IV) meso-tetrakis(3,5-
1
2531
3031
tw
dimethoxyphenyl)porphyrin (1)
16
Oxidovanadium(IV) β-octabromo-meso-
tetrakis(2,6-dibromo-3,5-
0.5
12227
14347
tw
dimethoxyphenyl)porphyrin (2)
aref. = reference no; bTOF = TON/reaction time in hour; tw = this work
Table 6. TOF (h-1) values for oxidative bromination of thymol/terpenes with different catalysts
known in literature.
S.No.
Catalyst
Reaction
time (h)
Maximum
TOF (h-1)
Ref.a
1
2
Dioxidomolybdenum(VI) complexes of
hydrazones.
2
1207
33
61
Vanadium(V) complexes of tripodal
ligands.
2
3568
3
4
5
6
Vanadium(V) complexes of Deferasirox.
Chemical catalysis using NH4VO3
Enzymatic catalysis
2
24
24
3
2475
15
62
63
63
73
1481
855
Oxidoperoxidotungsten(VI) and
dioxidotungsten(VI) complexes.
7
Oxidovanadium(IV) β-octabromo-
meso-tetrakis(2,6-dibromo-3,5-
dimethoxyphenyl)porphyrin (2)
1
83333
tw
aref. = reference no; tw = this work
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Conclusions
Oxidovanadium(IV) porphyrins 1 and 2 were synthesized in high yields and characterized by
various spectroscopic techniques and single crystal X-ray analysis. 1 and 2 exhibited good
thermal and chemical stabilities and were successfully utilized as catalysts for epoxidation of
olefins using H2O2/NaHCO3 as the oxidant and CH3CN/H2O as the reaction medium. 2 being
more efficient because of its electron deficient nature, steric crowding and nonplanarity was
found to be the best catalyst for oxidative bromination of thymol and other phenols while using
H2O2/KBr/HClO4 as brominating agent in water as well as for epoxidation reactions in aqueous
acetonitrile. In addition to showing good efficiency and high TOF numbers, both catalysts were
successfully recovered at the end of the reaction. Overall, the oxidovanadium(IV) porphyrin 2
has exhibited highest TOF numbers for the epoxidation of alkenes and oxidative bromination of
thymol and other phenols so far known in the literature among the different transition metal and
organometallic catalysts.
Experimental Section:
Chemicals and Materials:
Propionic acid was purchased from Thomas Baker, India and used as such. Thymol, phenol,
para-cresol, salicylaldehyde, 3,5-dimethoxybenzaldehyde and various alkenes used in our work
were purchased from Alfa Aesar, UK and used without any further processing. TBAPF6
(Tetrabutylammonium hexafluorophosphate) was also purchased from Alfa Aesar, UK and was
used after recrystallizing it two times from ethanol and vacuum drying. NaHCO3 and
VOSO4.xH2O were purchased from HiMedia, India and used without further purification.
Pyrrole was purchased from SRL Chemicals, India and used as received. Liquid bromine was
purchased from SD Fine chemicals, India and used as such. H2O2 and silica gel (100-200 mesh)
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were purchased from Rankem, India and used without further processing. CHCl3 was purchased
from Molychem, India and used after drying over P2O5. CH3CN and DMF of analytical grade
were purchased from Merck, India and used as received.
Instrumentation:
UV-Visible spectra were recorded using an Agilent Cary 100 spectrophotometer along with a
pair of quartz cells with 3.5 mL volume and 1 cm path length. IR spectra were recorded using a
mid IR range 4000-400 cm-1 Perkin-Elmer spectrophotometer by making KBr pellets. For
MALDI-TOF spectra, we have used Bruker UltrafleXtreme-TN MALDI-TOF-MS spectrometer
and HABA matrix. Cyclic voltammetric measurements were performed on a CHI 620E
instrument, using a three electrode assembly which consisted of Ag/AgCl reference electrode, Pt
disk working electrode and Pt-wire as the counter electrode along with TBAPF6 as the
supporting electrolyte. EPR spectra were recorded on Bruker EMX EPR spectrometer at 120 K
in toluene. NMR spectra were recorded in DMSO-d6 on a JEOL ECX 400 MHz spectrometer.
Single crystal X-ray diffraction data were collected on a Bruker APEX-II diffractometer with
APEX-II CCD detector and JAPAN thermal Engineering Co. Ltd. Cryo system, DX-CS190LD.
The crystal structures were solved by using direct methods (SHELXS-97). TGA/DTA was
carried out using a SII EXSTAR 6300 instrument. The products obtained from the catalyzed
reactions were identified using a Shimadzu 2010 plus gas-chromatograph endowed with an Rtx-1
capillary column (30 m × 0.25 mm × 0.25 μm) and an FID detector. The product identities were
confirmed using a Perkin-Elmer GC-MS (Clarus 500).
Synthesis of Oxidovanadium-5,10,15,20-tetrakis(3,5-dimethoxyphenyl)porphyrin (1):
Free base 5,10,15,20-tetrakis(3,5-dimethoxyphenyl)porphyrin (500 mg, 0.584 mmol) was
dissolved in 100 mL DMF. To this, 10 equiv. of VOSO4 (962 mg, 5.85 mmol) was added. It was
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stirred for 10 minutes and then refluxed under inert atmosphere for 16 h. The reaction mixture
was cooled, after the completion of the reaction, to room temperature. Distilled water (150 mL)
was added afterwards which led to the formation of a precipitate. It was filtered using a G4
crucible and the precipitate vacuum dried for 6 h. The dried, crude product was redissolved in
CHCl3 and purified over a silica column using CHCl3 as eluent. The yield was found to be 96%
(514 mg, 0.559 mmol). UV/Vis (CH2Cl2): λmax (nm) (log ε): 425(5.57), 547(4.28). MALDI-TOF-
MS (m/z): found 920.171 [M+H]+, calcd. 919.25. Anal. Calcd. for C52H44N4O9V: C, 67.90; H,
4.82; N, 6.09. Found: C, 67.02; H, 4.46; N, 6.02.
Synthesis
of
Oxidovanadium-β-octabromo-meso-tetrakis(2,6-dibromo-3,5-
dimethoxyphenyl)-porphyrin (2):
1 (300 mg, 0.326 mmol) was dissolved in 150 mL distilled CHCl3. 82 Equiv. of liq. Br2 (1.35
mL, 26.73 mmol) was dissolved in 20 mL distilled CHCl3 and added drop wise to the above
solution over a period of 45 minutes with constant stirring. Stirring was continued to a further of
4 h. After this period, pyridine 63 equiv. (1.62 g, 20.53 mmol) dissolved in 5 mL distilled CHCl3
was added drop wise over 30 minutes. Stirring was continued for another 12 h. At the end of this
period, the reaction mixture was washed with a saturated sodium metabisulphite solution
followed by washing with dil. HCl (2 times) and distilled water (2 times). The organic layer was
extracted and dried over anhydrous Na2SO4. The solvent was removed over a rotary evaporator.
The dried, crude product was redissolved in CHCl3 and purified over a silica column using
CHCl3 as eluent. This reaction yield was found to be 83% (590 mg, 0.270 mmol). UV/Vis
(CH2Cl2): λmax (nm) (log ε): 463 (5.31), 592 (4.24). MALDI-TOF-MS (m/z): found 2179.60
[M−2H]+, calcd. 2181.81. Anal. Calcd. for C52H28Br16N4O9V: C, 28.62; H, 1.29; N, 2.57.
Found: C, 28.32; H, 1.14; N, 2.33.
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Catalytic Epoxidation of Alkenes
The catalytic efficacy of 1 and 2 towards selective epoxidation of alkenes was evaluated using
1 as the representative catalyst. In a typical catalytic reaction, 5 mmol styrene (0.52 g), 15 mmol
30% H2O2 (1.69 g) and 1.5 mmol NaHCO3 (0.126 g) were mixed in aqueous MeCN (3:2, v/v) in
a 50 mL two neck round bottom flask fitted with a reflux condenser. Catalyst 1 (0.0015 g, 1.63
µmol) was added to the reaction mixture, and it was stirred with 500 rpm at 60 ᴏC on an oil bath
for 1 h. The progress of the reaction was monitored by withdrawing small aliquots of the reaction
mixture at every 15 minute intervals and extracting them vigorously with hexane. The hexane
layer was then analyzed by injecting ca. 0.2 µL into GC/GC-MS. The oxidation products were
quantified using a Shimadzu 2010 plus gas-chromatograph equipped with an Rtx-1 capillary
column (30 m × 0.25 mm × 0.25 μm) and an FID detector. The product identities were confirmed
by using a Perkin-Elmer GC-MS (Clarus 500). The effect of various reaction parameters such as
amount of catalyst, amount of oxidant, amount of NaHCO3, amount of solvent, type of solvent
and temperature on the final conversion was also assessed in order to obtain the best optimized
reaction conditions for maximum conversion.
First, the solvent ideal for catalytic activity was determined by performing the reactions in 5
mL each of different solvents such as MeCN, H2O, MeCN/H2O as well as without any solvent,
for 5 mmol (0.52 g) styrene, 15 mmol (1.69 g) 30% H2O2, 2 mmol (0.168 g) NaHCO3, 0.0015 g
(1.63 µmol) of 1 at 60 ˚C for 1 h. It was observed that while in MeCN, a total conversion of 77%
was achieved, no reaction occurred in any other solvent. A mixture of aqueous and organic
solvents is known to accelerate the epoxidation reaction.30,44 Thus, the effect of water dilution
was studied by considering three different ratios of MeCN:H2O mixtures, i.e. 2:1, 3:2 and 4:3.
Indeed, dilution improved the conversion to 81% for 2:1 ratio, and further to 90% for 3:2 ratio,
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but more dilutions with water did not improve the conversion significantly. Thus, a mixture of 3
mL MeCN and 2 mL H2O was chosen as the best solvent to achieve maximum conversion (entry
no. 3, Table 1).
Next, the amount of catalyst was optimized. To achieve this, three parallel reactions were
performed taking different amounts of the catalyst, viz. 0.0010 g, 0.0015 g and 0.0020 g, while
keeping all other parameters constant, i.e. 5 mmol (0.52 g) styrene, 15 mmol (1.69 g) 30% H2O2,
2 mmol (0.168 g) NaHCO3, 5 mL MeCN/H2O (3:2) at 60 ˚C for 1 h. The conversion
significantly improved from 74% to 90% on increasing the catalyst amount from 0.0010 g to
0.0015 g. However, no further increment in conversion was observed upon further increasing the
catalyst amount, for example, to 0.0020 g (91%). Thus, the catalyst amount 0.0015 g (1.63 μmol)
was considered as the optimal catalyst amount for obtaining the best conversion (entry no. 3,
Table 1).
The effect of oxidant amount, 30 % H2O2 for 5 mmol (0.52 g) of styrene in the presence of
0.0015 g of catalyst and 2 mmol (0.168 g) of NaHCO3 was determined by performing the
reactions in different styrene to H2O2 molar ratios of 1:1 (5 mmol (0.565 g) 30 % H2O2), 1:2 (10
mmol (1.13 g) 30 % H2O2) and 1:3 (15 mmol (1.69 g) 30 % H2O2) in 5 mL MeCN/ H2O (3:2) at
60 ˚C for 1 h. The lowest conversion of 15% was achieved using 5 mmol of 30% H2O2.
However, the conversion significantly improved to 78% and 90% for 10 mmol and 15 mmol of
30% H2O2 respectively, indicating that 15 mmol of 30% H2O2 was required to achieve maximum
reaction conversion (entry no. 3, Table 1).
Next, the amount of promoter, NaHCO3, was optimized to obtain maximum reaction
conversion. For this, three different amounts of NaHCO3; 1 mmol (0.084 g), 1.5 mmol (0.126 g)
and 2 mmol (0.168 g) were taken, keeping all other factors constant such as 5 mmol (0.52 g)
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styrene, 15 mmol (1.69 g) 30% H2O2, 0.0015 g (1.63 μmol) of 1 at 60 ˚C. After 1 h of reaction
time, while an increase in conversion from 77% to 89% was achieved on increasing the NaHCO3
amount from 1 mmol to 1.5 mmol, further increment in NaHCO3 amount did not affect the final
conversion and 2 mmol of NaHCO3 gave 90% conversion. Thus 1.5 mmol of NaHCO3 was
considered as the optimum amount (entry no. 10, Table 1).
The dependence of reaction on temperature was determined by performing the reaction at three
different temperatures, viz. 40 °C, 50 °C and 60 °C under the conditions optimized above for 1 h.
While at 40 °C, only 34% conversion was achieved, raising the temperature to 50 °C
significantly improved the conversion to 87% in 1h (entry no. 11, Table 1). At 60 °C a
conversion of 89% was achieved in 1h.
Vanadium Haloperoxidases Activity: Oxidative Bromination of Thymol
Vanadium haloperoxidases type activity towards oxidative bromination of thymol and other
phenols was evaluated using the more active catalyst 2. In a typical reaction, thymol (1.5002 g,
10 mmol), KBr (2.38 g, 20 mmol), H2O2 (2.2665 g, 20 mmol) and water (20 mL) were taken in a
50 mL reaction flask. Catalyst (0.00025 g, 0.114 µmol) and HClO4 (0.717 g, 0.5 mmol) were
added and the mixture was stirred at room temperature. An additional 1.5 mmol of HClO4 was
added in three equal portions at intervals of fifteen minutes. After 2 h the reaction mixture was
extracted with diethyl ether (or hexane), washed with brine (NaCl) solution and dried over
anhydrous Na2SO4. The reaction products were separated using a silica gel column. The products
were analyzed and identified by GC, GC–MS and 1H NMR.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on RSC website.
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Synthetic procedures, UV-Vis and MALDI-TOF-MS spectra, crystal structure data,
FMOs of synthesized compounds and Cyclic Voltammograms (PDF)
Accession Codes
CCDC 1861933 and 1861934 contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
E-mail: sankafcy@iitr.ac.in; Tel: +91-1332-284753; Fax: +91-1332-273560
E-mail: rkmanfcy@iitr.ac.in; Tel: +91-1332-285723; Fax: +91-1332-273560
ORCID
Muniappan Sankar: 0000-0001-6667-3759
Mannar R. Maurya: 0000-0001-9977-6287
Bhawna Uprety: 0000-0002-5305-3726
Notes
The authors declare no competing financial interests.
§Author present address
Department of Chemistry, University of Johannesburg, Auckland Park 2006, Johannesburg,
South Africa.
ACKNOWLEDGMENT
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TAD and UP are thankful to MHRD, India for senior research fellowship. We sincerely thank
Science and Engineering Research Board (EMR/2016/004016) for financial assistance. We are
really grateful to Reshu, Nancy, Abhilasha and Shaliendra for their help in the catalytic studies.
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