Selective Bromination of β-Positions of Porphyrin by Self-Catalytic Behaviour of VOTPP: Facile Synthesis, Electrochemical Redox Properties and Catalytic Application

Article abstract of DOI:10.1002/ejic.202100116

Oxidovanadium(IV) complex of β-octabromo-meso-tetraphenylporphyrin, {[V^IVO(TPPBr₈)], 2} was synthesized by self-catalytic oxidative bromination of meso-tetraphenylporphyrinatooxidovanadium(IV) {[V^IVO(TPP), 1} in excellent yield under mild conditions at 25 °C and its structure was confirmed by single crystal X-ray study. UV-Vis and ¹H NMR spectra further confirmed that the meso-phenyl rings are not brominated and thus emphasizes on the selectivity as well as synthetic importance of this catalytic method. In the presence of substrates e. g. phenol derivatives, 1 biomimics the vanadium bromoperoxidase (VBPO) enzyme and catalyses the oxidative bromination of substrates in water at 25 °C. Remarkably, 2 also catalyses the oxidative bromination of phenol derivatives under similar reaction conditions with very high turnover frequency (TOF) values (ca. 29 s^?1) along with its recyclability. It was found that 2 showed superior catalytic performance as compared to 1 because of its electron deficient nature due to electron withdrawing bromo substituents and robust saddle shaped nonplanar structure (further supported by DFT studies).

Full text of DOI:10.1002/ejic.202100116

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Selective Bromination of β-Positions of Porphyrin by Self-
Catalytic Behaviour of VOTPP: Facile Synthesis,
Electrochemical Redox Properties and Catalytic Application
Mannar R. Maurya,*[a] Ved Prakash,[a] Fernando Avecilla,[b] and Muniappan Sankar*[a]
Oxidovanadium(IV) complex of β-octabromo-meso-tetraphenyl-
porphyrin, {[VIVO(TPPBr8)], 2} was synthesized by self-catalytic
oxidative bromination of meso-tetraphenylporphyrinatooxido-
vanadium(IV) {[VIVO(TPP), 1} in excellent yield under mild
bromoperoxidase (VBPO) enzyme and catalyses the oxidative
°
bromination of substrates in water at 25 C. Remarkably, 2 also
catalyses the oxidative bromination of phenol derivatives under
similar reaction conditions with very high turnover frequency
(TOF) values (ca. 29 sÀ 1) along with its recyclability. It was found
that 2 showed superior catalytic performance as compared to 1
because of its electron deficient nature due to electron with-
drawing bromo substituents and robust saddle shaped non-
planar structure (further supported by DFT studies).
°
conditions at 25 C and its structure was confirmed by single
crystal X-ray study. UV-Vis and 1H NMR spectra further
confirmed that the meso-phenyl rings are not brominated and
thus emphasizes on the selectivity as well as synthetic
importance of this catalytic method. In the presence of
substrates e.g. phenol derivatives, 1 biomimics the vanadium
Introduction
To the best of our knowledge, we did not find any report on
the effect of bromide ions on the catalyst itself in the absence
of substrate (self-catalysis) under suitable catalytic conditions.
Perhaloporphyrins and corroles are useful catalysts for
various organic transformations and oxygen reduction
reactions.[14–18] Recently, Conte and coworkers have reported the
bromination of [MII(TPP)] [M=Ni, Cu and Zn] to get [MII(TPPBr8)]
using KBr in the presence of 100 mol% vanadium(V) or
molybdenum(VI) catalysts.[19] The yields were found to be lower
(22–25%) except for [CuII(TPP)] and also yields mixture of
products in some cases.[19] Recently, we observed that common
reagents for oxidative bromination i.e. KBr, H2O2, and HClO4 in
the presence of vanadyl porphyrins in water brominate thymol
and other phenol derivatives efficiently and the catalysts are
recyclable.[18] However, there is no report on direct bromination
of [VIVO(TPP)] (1) to obtain [VIVO(TPPBr8)] (2). In general, 2 has
been synthesized from [CuII(TPP)] through a multistep synthesis
as shown in Scheme 1.[20] Herein, we report the one-step self-
catalytic reaction of 1 to obtain 2 in excellent yield and its
utilization as a catalyst for the oxidative bromination (biomimics
of V-HPO) in water. We observed that in the absence of any
phenol derivatives (i.e. substrate for oxidative bromination), the
catalyst, 1 brominates itself at β-positions of the porphyrin
macrocycle to give 2 in 96% yield under the reaction conditions
usually applied for the oxidative bromination at room temper-
ature. Details of optimization of the catalytic reaction, isolation
of 2 and its characterization including single crystal X-ray
structure are reported herein. Further, 2 was utilized for the
oxidative bromination of phenol derivatives with very high TOF
values.
Vanadium is a trace element which is widely distributed in
nature and found to be 100 ppm in the Earth’s crust and 35–
50 nM in the ocean.[1] Till date, only two classes of vanadium
containing enzymes have been isolated: (i) vanadium-depend-
ent haloperoxidases (V-HPO) and (ii) vanadium nitrogenases.[2]
Oxidative halogenation is one of the key reactions shown by
enzyme V-HPO.[2,3] From biomimetic chemistry point of view,
such reactions are very important as functional models where
the ligated vanadate ion oxidizes halide ions in the presence of
oxidant such as H2O2 to produce hypohalous acid (HOX) which
are chemically equivalent to an electrophilic “X+”.[4] During
oxidative bromination, as observed in enzymatic system as well
as functional models, an oxido(peroxido)vanadium(V) species
forms which allows nucleophilic attack of the bromide ion,
followed by release of HOBr/Br2 which actually brominates the
substrates.[5–9] A good number of research papers dealing with
‘‘green” procedures for introducing bromo group(s) into organic
molecules,[10–13] a reaction of primary synthetic and industrial
importance, has been analysed by many research groups.[1–13]
[a] Prof. Dr. M. R. Maurya, V. Prakash, Prof. Dr. M. Sankar
Department of Chemistry,
Indian Institute of Technology Roorkee,
Roorkee – 247667, India
E-mail: rkmanfcy@iitr.ernet.in
sankafcy@iitr.ac.in
https://www.iitr.ac.in/CY/rkmanfcy
https://www.iitr.ac.in/~CY/sankafcy
[b] Prof. Dr. F. Avecilla
Grupo Xenomar, Centro de Investigacións Científicas Avanzadas (CICA),
Departamento de Química, Facultade de Ciencias,
Universidade da Coruña, Campus de A Coruña,
15071 A Coruña, Spain
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejic.202100116
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Scheme 1. Literature method for the synthesis of [VIVO(TPPBr8)] (2).[20]
IV
IV
°
Scheme 2. Synthesis of [V O(TPPBr8)] (2) by self-catalytic reaction of [V O(TPP)] (1) using HClO4/KBr/H2O2 at 25 C.
Results And Discussion
Synthesis of [VIVO(TPPBr8)] (2) via Self-catalysed Oxidative
Bromination of [VIVO(TPP)] (1) and its Stability
The self-catalysed oxidative bromination at β-positions of
pyrrole ring in [VIVO(TPP)] (1) was carried out at room temper-
ature in the presence of KBr, 30% aqueous H2O2 and 70%
aqueous HClO4 in a biphasic reaction condition (H2O:CH2Cl2,
5:2, 7 mL) (Scheme 2). The reaction was investigated by
changing different parameters that may affect the rate of
bromination. Thus, for 0.05 mmol (34.0 mg) of [VIVO(TPP)] (1),
different amounts of KBr (0.5, 0.75, 1.0, 1.5, 2.0, 2.5 and
3.0 mmol), different amounts of 30% aqueous H2O2 (0.5, 0.75,
1.0, 1.5, 2.0, 2.5 and 3.0 mmol) and different amounts of 70%
aqueous HClO4 (0.5, 0.75, 1.0 and 1.25 mmol) were considered
and the UV-visible spectral changes were monitored (Figure 1).
Figure 1. UV-Vis spectra recorded during oxidative bromination of [VIVO-
(TPP)] (1) under different reaction conditions in CHCl3. Inset of the figure
presents different KBr:H2O2:HClO4 ratios used for each set of reaction
conditions with respect to 1 equivalent of 1.
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Since, the aromatic rings, pyrrole and phenyl moieties, present
in the porphyrin units are not in the same plane relative to each
other (vide infra), the π-clouds in the complex are not affected
much so they do not modify Soret band and thus monitoring of
spectral changes became easy. The reaction was monitored
through TLC and the final brominated product under each
specified reaction condition was purified by column chromatog-
raphy and characterized by UV-visible spectroscopy. Table 1
presents the details of reaction conditions and UV-visible
spectral data found in CHCl3 under each specified condition.
The optimized condition for complete bromination of
0.05 mmol of [VIVO(TPP)] (1) requires 2.5 mmol each of KBr and
H2O2 and 1 mmol of HClO4 at room temperature (Table 1) to
obtain 96% yield of 2. Complex 2 is soluble in CH2Cl2, CHCl3,
MeCN, benzene, toluene, DMF and DMSO but insoluble in
water.
Figure 1 presents UV-visible spectral changes for various
brominated products under different reaction conditions in
CHCl3 and in few specific cases the products were confirmed by
their m/z peaks using MALDI-TOF-MS. The mass spectrum of
the reaction mixture after 30 minutes indicated a mass peak at
711.74 m/z (Figure S1) and thus supports the formation of
oxidoperoxido species which is getting protonated in the
presence of acid. It is clear from the Figure 1 that increasing the
KBr:H2O2:HClO4 ratio form 10:10:10 to 50:50:20 for the fixed
amount of 1 (34.0 mg, 0.05 mmol) slowly changes the position
of Soret band from 422 nm to 464 nm (5 nm red-shift per
bromo substituent) with considerable broadening. Simultane-
ously, the 546 nm band shifts to higher wavelength to 590 nm.
Notably, the wavelenth 585 nm, a longest Qx(0,0) band, intensifies
and red-shifts to 628 band along with the appearance of a new
band at 362 nm under the specified reaction conditions (entry 8
of Table 1). The reaction was also monitored at different time
interavels from 1 h to 36 h (Table S1). It was found that the
reaction requires 36 h to form the expected brominated
product i.e. [VIVO(TPPBr8)] (2)[20] selectively (Figure S2). Hence,
we have optimized the reaction conditions for the selective
Scheme 3. Plausible catalytic mechanism for the self-catalytic bromination
reaction of [VIVO(TPP)] (1).
oxidoperoxido species is possibly responsible for the produc-
tion of HOBr which facilities the bromination of [VIVO(TPP)]
(1).[21,22] The progress of the reaction was also monitored using
1H NMR spectroscopy (Figure 2). The disappearance of β-
protons signal indicates that they are all brominated. Molecular
mass (observed 1310.04 m/z, calc. 1310.45), obtained for the
product from MALDI-TOF-MS (Figure S3). FTIR spectra of 1 and
2 exhibit a prominent νV=O peak at 1002 and 1028 cmÀ 1,
respectively (Figure S4 and S5) which is not observed in
H2TPPBr8 (Figure S6). Further, 2 exhibits the absorption spectral
maxima (λmax) at 362, 464, 590 and 628 (s)nm in CHCl3 at 298 K
as shown in Figure 1 which are in accordance with reported
literature values for 2.[20] During the oxidative bromination
reaction, the pH of the reaction goes down to ca. 1.5, whereas
to our surprise, the complex is stable and does not undergo any
decomposition. Complex 2 was further characterized by 1H NMR
°
oxidative bromination of 1 at ambient temperature (ca. 25 C)
and isolated β-octabromo-meso-tetraphenylporphyrinatooxido-
vanadium(IV) (2) in 96% yield. Conte et al. and others have
suggested the formation of oxidoperoxido intermediate during
the oxidative bromination reaction[10c,f,11–13,19] as shown in
Scheme 3 which is in line with our studies. Further, it gets
protonated in the presence of HClO4.[21,22] The protonated
1
spectroscopy in CDCl3 at 298 K. In general, the H NMR spectra
Table 1. Results for the optimization of bromination of [VIVO(TPP)] (1) (34.0 mg, 0.05 mmol) in biphasic reaction condition (H2O:CH2Cl2, 5:2, 7 mL) at room
temperature and the UV-visible spectral data recorded in CHCl3 for each set of reaction conditions.
Sr. No.
Catalyst
[mmol]
KBr
[mmol]
H2O2
[mmol]
HClO4
[mmol])
Time
[h]
UV-vis (λmax
[nm])
1.
2.
3.
4.
5.
6.
7.
8.
9
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
05
0
0
0
0
310, 422, 546, 585
333, 433, 558, 602
343, 444, 568
350, 455, 582
353, 457, 582
357, 460, 587
360, 462, 589
362, 464, 590, 628
358, 462, 587
0.5
0.75
1.0
1.5
2.0
2.5
2.5
3.0
0.5
0.75
1.0
1.5
2.0
2.0
2.5
3.0
0.5
0.75
1.0
0.75
1.0
1.0
1.0
1.25
20
36
36
36
36
36
36
36
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Figure 2. 1H NMR spectra of [VIVO(TPP)] (1) during the bromination reaction at different time intervals (1 to 36 h). Reagents: 0.05 mmol of 1, 2.5 mmol of KBr
and aqueous 30% H2O2 in the presence of 1 mmol of 70% aqueous HClO4. About 10 μL of organic layer was taken out from the reaction mixture, extracted
further with CH2Cl2, evaporated the solvent in vacuum and then 1H NMR was recorded in CDCl3.
°
of vanadyl porphyrins exhibit line broading due to papramag-
netic nature of V(IV) metal centre (d1 configuration).[22b] 1H NMR
spectrum of 2 in CDCl3 at 298 K (Figure 2) shows only three
broad signals at 8.30, 7.93 and 7.66 ppm which corresponds to
o-, m- and p-protons of meso-phenyl ring with the protons
intensity of 8, 8 and 4, respectively (Figure S7). There is no
signal for the β-pyrrole protons in the range of 8.8 to 9.8 ppm
in the isolated product 2 which confirms that all the β-positions
are brominated.[22b] Hence, UV-visible and 1H NMR spectra
confirm that the meso-phenyl rings are not getting brominated
and thus emphasizing on the selectivity as well as synthetic
importance of this catalytic method.
into the saturated toluene and chloroform solution at 25 C. The
crystal data and refinement details are given in the experimen-
tal section. Figure 3 shows ORTEP representations of 2.
Compound 2 crystalizes in a centrosymmetric space group, I41/
a. The asymmetric unit only contains a quarter of the porphyrin
in the compound of formula C44H20N4OVBr8. Two positions are
observed for fragment VO, which are placed to 0.4662(2) Å from
the N4 porphyrin plane (Figure S9). The selected bond lengths
and bond angles are listed in Table 2. Complex 2 has a
nonplanar saddle shape conformation with Δ24 (average mean
It is important to mention here that bromination of β-
positions of [VIVO(TPP)] using even 16 equivalents of NBS,
against 12 equivalent used for such bromination in case of
[NiII(TPP)],[23] did not show any significant result. Similarly,
bromination of [VIVO(TPP)] using 40 equivalents of liquid
bromine, against 32 equivalents used for such bromination in
case of [CuII(TPP)],[20] gave a mixture of partially brominated
products (Figure S8).
IV
°
Table 2. Bond lengths (Å) and angles ( ) for [V O(TPPBr8)] (2).
Bond lengths
Experimentally observed
Obtained from DFT study
Br(1)À C(2)
V(1)À O(1)
V(1)À N(1)
N(1)À C(4)
N(1)À C(1)
Br(2)À C(3)
1.874(7)
1.616(11)
2.040(5)
1.376(8)
1.387(8)
1.869(6)
1.929(7)
1.594(11)
2.066(5)
1.401(8)
1.401(8)
1.929(6)
Bond angles
O(1)À V(1)À N(1)
C(4)À N(1)À C(1)
C(3)À C(2)À Br(1)
C(1)À C(2)À Br(1)
C(2)À C(3)À Br(2)
C(4)À C(3)À Br(2)
101.38(19)
107.7(5)
123.4(5)
127.2(5)
124.4(5)
127.9(5)
101.0(19)
107.8(5)
123.9(5)
127.9(5)
123.9(5)
127.9(5)
Structure Description of [VIVOTPPBr8)] (2)
Compound [VIVO(TPPBr8)] (2) crystallizes as black plates (crystal
dimensions: 0.1×0.07×0.03 mm) by slow diffusion of methanol
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phenyl moieties, present in the porphyrin units not in the same
plane relative to each other and non-planar saddle shape
conformation. This non-planarity prevents interactions by π-π
staking in the whole structure, between different complexes.
The π-clouds of these moieties are thus not affected much.
DFT Studies
The geometry optimization was carried out using the B3LYP
functional with the LANL2DZ basis set. The optimized geo-
metries (top and side views) of 2 are shown in Figure 4. 2
exhibits nonplanar saddle shape confirmation due to steric
hindrance between β-bromo and meso-phenyl groups which
results two pyrroles up and two pyrroles down to the porphyrin
mean plane. The average mean plane deviation of 24 macro-
cycle core atoms (Δ24)=0.589 Å and the average mean plane
deviation of β-pyrrole carbons (ΔCβ)= �1.235 Å confirm the
saddle shape confirmation of the macrocyclic core. The vanadyl
ion is located above the plane by about 0.552 Å. The bond
distances V=O and CβÀ Cβ in the case of 2 were found to be
1.594 Å and 1.383 Å, respectively, and are in agreement with
the single crystal X-ray studies (Table 2). Hence, the structural
robustness of 2 is due to highly nonplanar structure which
prevents the formation of μ-oxido dimer as well as the
oxidation of macrocyclic ring by H2O2. Other calculated bond
lengths and angles (Table 2) are also very close to the
calculated ones.
Figure 3. ORTEP for the complex [VIVO(TPPBr8)] (2). All the non-hydrogen
atoms are presented by their 40% probability ellipsoids.
plane displacement of 24 core atoms) value of 0.599 Å and ΔCβ
(average mean plane displacement of β-pyrrole carbon atoms)
value of �1.1615 Å. The bond distances V=O and CβÀ Cβ of 2
were found to be 1.616 Å and 1.356 Å, respectively. The bond
distance of V=O in 2 is slightly shorter than the reported for
[VIVO(TPP)] [1.625(16) Å][24] while the distances VÀ N of the
pyrrole rings are in normal range observed for this type of
compounds.[24,25] In the refinement of the structure, we observe
an anomalous behavior of this fragment. Pyrrole and phenyl
EPR Studies
X-band EPR spectrum of 2 in toluene was recorded to confirm
the +4 oxidation state of vanadium (Figure 5). The eight lines
axial EPR spectrum of 2 corresponds to I=7/2 value of the
vanadium nucleus. From the anisotropic EPR spectrum of 2 the
°
rings form dihedral angles of 66.8 , and dihedral angle between
°
pyrrole rings is 33.4 , resulting to aromatic rings, pyrrole and
Figure 4. B3LYP/LANL2DZ optimized geometry of [VIVO(TPPBr8)] (2) showing (a) the top view and (b) side view. In the side view, the meso-ph substituents are
not shown for clarity.
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Cyclic Voltammetry Studies
Figure 6 presents cyclic voltammograms (vs. Ag/AgCl) of 1 and
2 in CH2Cl2 containing 0.1 M TBAPF6 at 298 K. In general,
porphyrins show two reversible oxidations and reductions
during cyclic voltammetric studies.[16,18,28–31] Similarly, 1 and 2
show two reversible porphyrin ring-centred one electron
oxidations and reductions.[30] This was further confirmed by
differential pulse voltammetric study in case of [VIVOTPPBr8] (2)
in CH2Cl2 (Figure S10). Interestingly, complex 2 exhibits 190 mV
anodic shift in first oxidation and 440 mV in the first reduction
potential as compared to 1 due to the electron withdrawing
bromo substituents and nonplanar porphyrin core.[19,20,28–31,32]
Hence, 2 is highly electron-deficient compared to 1 and other
meso-tetraphenylporphyrins (MTPPs). The electron deficient
nature of 2[32] is an additional advantage for the better catalytic
activity. Since there is no prominent peak in the range of 0.75
to À 0.75 V, no metal-centred redox reaction occurs in 1 and
2.[30] The reported porphyrin ring redox potentials are in line
with the reported literature for vanadyl porphyrins by Conte et.
al.[30] and Kadish et al..[31]
Figure 5. X-band EPR spectrum of [VIVO(TPPBr8)] in toluene at 100 K.
gk and g? were depicted as 1.959 and 1.987, respectively,
whereas Ak and A? were obtained as 167.1 × 10À 4 and 74.4 ×
10À 4 cmÀ 1, respectively. All these values are within the permis-
sible range for an axially compressed V(IV) d1 system.[16,18,26,27]
Catalytic Activity Study – Oxidative Bromination of Phenol
Derivatives
We have carried out the oxidative bromination of various
phenol derivatives using [VIVO(TPP)] as catalyst in order to
understand if it also undergoes bromination in the presence of
substrate i.e. phenol. Thus, for 10 mmol (940.0 mg) of phenol,
following parameters were varied to optimize the reaction
conditions: (i) three different amounts of catalyst [VIVO(TPP)]
(0.25, 0.50 and 1.0 mg), (ii) three different amounts of KBr (10,
20 and 30 mmol), (iii) three different amounts of oxidant 30%
H2O2 (10, 20 and 30 mmol), and (iv) two different amounts of
70% aqueous HClO4 (10 and 20 mmol) and reaction was carried
out in water at room temperature. Table S2 presents the details
of conversion under each specified condition.
The best suited reaction conditions for the maximum
conversion of 10 mmol of phenol were found to be: catalyst
[VIVO(TPP)] (0.25 mg, 0.368 μmol), KBr (2.38 g, 20 mmol), aque-
ous 30% H2O2 (2.027 mL, 20 mmol), aqueous 70% HClO4
(1.713 mL, 20 mmol, added in four equal portions of 0.428 mL,
each at interval of 5 minutes) and water (20 mL) (entry 11,
Table S2). Notably, reaction reaches to 100% conversion under
this reaction conditions within half an hour with a TOF value of
15.1 sÀ 1. It is important to note that lower canversion ended
with the formation of only two major products, o-bromophenol
and p-bromophenol while 100% conversion led to the
formation of an additional product 2,4-dibromophenol along
with two initial ones [Figure S11 and S12)]. Since there was no
change in colour of the complex along with no indication of
the bromination of [VIVO(TPP)] in the presence of substrates,
nature of UV-visible spectrum remained the same (Figure S13).
Thus, the self bromination of [VIVO(TPP)] is selective in the
absence of substrates.
Figure 6. Cyclic voltammograms (vs. Ag/AgCl) of [VIVO(TPP)] (1) and [VIVO-
(TPPBr8)] (2) in CH2Cl2 containing 0.1 M TBAPF6 with the scan rate of 100 mV/
s at 298 K.
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Complex 1 was also used for the oxidative bromination of
as proposed in Scheme 3 during catalytic bromination of
phenols.
Though overall conversions of substrates and the selectivity
other phenol derivatives (Table 3) under the similar reaction
conditions optimized for the phenol. In general, conversion of
most phenols are between 82–100% with one or two bromo
products. Since phenol is o- and p-directing, the phenol
derivatives such as p-cresol, 2,4-dimethylphenol, 2,6-dimeth-
ylphenol, 2,4-di-tert-butylphenol, 2-tert-butyl-4-methylphenol,
2-hydroxy-3,5-di-tert-butylbenzaldehye and 2,4-dichlorophenol
having any such position occupied, show single brominated
derivatives selectively. However, those phenol having o- and p-
positions free such as thymol, salicylaldehyde, 4-hydoxy-
benzaldehyde, 4-tert-butylphenol, gave both o- and p-bromo
derivatives. In case of p-cresol and thymol, an additional
product dibromo derivative was also obtained. It is clear from
the Table 3 that electron donating group(s) enhance(s) the
reactivity of phenol which in turn influence on the relatively
higher yield of products. However, there are no systematic
trend on the selectivity of different products where more than
one bromo-derivatives are formed but over all a good TOF was
observed in all cases.
Complex 2 was also utilized for the oxidative bromination of
phenol derivatives (Table 4). Again, all parameters were opti-
mised as mentioned for catalyst 1. Thus, for 10 mmol of phenol,
20 mmol of KBr, 20 mmol of aqueous 30% H2O2 and 20 mmol
of aqueous 70% HClO4 (added in four equal portions as
menthoned above) in 20 mL of water, only 0.0019 mmol% of 2
was required to get best conversion. During the course of
reaction, we observed mass peaks at m/z 1342.32 and 1360.25
corresponding to the formation of oxidoperoxido and oxidoper-
oxido along with water adduct of [VIVO(TPPBr8)] (2), respectively,
during oxidative bromination reaction (Figure S14) which fur-
ther suggests that the formation of oxidoperoxido intermediate
of obtained products do not differ much, the use of consid-
erably lower amount of 2 makes it better catalyst than 1
possibly due to its robust nonplanar saddle shape structure and
electron withdrawing Br substituents.[16,18,28–31,33] The steric
repulsion between bromo and phenyl groups of 2 leads to
saddle shape nonplanar conformation (vide supra).[28–31] The
steric crowding around the porphyrin periphery also prevents
the μ-oxido dimer formation and oxidative ring opening of
porphyrin macrocycle. These results are also in line with the
anodic shift in redox potentials observed in CV studies as
well.[27] Hence, the proposed influence/change on the reaction
mechanism (as presented in Scheme 3) increases the catalytic
efficiency of 2.
Among various phenols, catalytic oxidative bromination of
thymol (2-isopropyl-5-methylphenol) is the most studied reac-
tion by vanadium compounds among the limited reports
available in the literature (Table 5). Generally conversion with all
catalysts are good (80–100%) but the over all turnover
frequencies are low as relatively higher amount of catalysts as
well as extended time to acquire equilibrium were required in
most cases. The oxidovanadium(iv) complex of β-octabromo-
meso-tetrakis(2,6-dibromo-3,5-dimethoxyphenyl)porphyrin,
however, shows much better result (98% conversion) with TOF
value of 24 sÀ 1 in catalysing oxidative bromination of diferent
phenols in 1 h of reaction time. However, catalysts reported
herein are still better as they complete reaction in 0.5 h with
better TOFs.
The UV-Visible spectra of 1 and 2 after recovering them
from the catalytic reaction mixtures after the first catalytic cycle
Table 3. Oxidative bromination of phenol derivatives using [VIVO(TPP)] (1) as catalyst in 20 mL water after 0.5 h of reaction time at room temperature.
Substrate
Phenol
Conversion [%]
95
TOF
Products
[% selectivity]
[a]
[sÀ 1
]
14.3
o-Bromophenol (21)
p-Bromophenol (31)
Dibromophenol (48)
Bromo-p-cresol (79)
Dibromo-p-cresol (21)
o-Bromothymol (9)
p-Cresol
99
95
14.9
14.3
Thymol
p-Bromothymol (46)
Dibromothymol (45)
Bromosalicylaldehyde (87)
Salicylaldehyde
92
13.9
10.6
15.1
Dibromosalicylaldehyde (13)
2-Bromo-4-hydroxybenzaldehyde (80)
2,6-Dibromo-4-hydroxybenzaldehyde (20)
2-Bromo-4-tert-butylphenol (56)
2,6-Dibromo-4-tert-butylphenol (44)
2-Bromo-4,6-dimethylphenol (100)
4-Bromo-2,6-dimethylphenol (100)
2,4-Di-tert-butyl-6-bromophenol (100)
2-Bromo-4-methyl-6-tert-butylphenol (100)
2-Hydroxy-3,5-di-tert-butyl-4,6-dibromo-benzaldehye (100)
2-Bromo-4,6-dichlorophenol (100)
4-Hydoxy-benzaldehyde
4-tert-Butylphenol
70
100
2,4-Dimethylphenol
2,6-Dimethylphenol
2,4-Di-tert-butylphenol
2-tert-Butyl-4-methyl-phenol
2-Hydroxy-3,5-Di-tert-butylbenzaldehye
2,4-Dichlorophenol
100
80
98
96
84
89
15.1
12.1
14.8
14.5
12.7
13.4
[a] TOF (turnover frequency): moles of substrate converted per mole of catalyst per second.
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Table 4. Oxidative bromination of phenol derivatives using [VIVO(TPPBr8)] (2) as catalyst in 20 mL water after 0.5 h of reaction time at room temperature.
Substrate
Conversion
(%)
TOF
(sÀ 1
Products
(% selectivity)
)
Phenol
p-Cresol
Thymol
95
96
90
27.7
28.0
26.2
p-Bromophenol (56),
o-Bromophenol (44)
Bromo-p-cresol (76),
Dibromo-p-cresol (24)
o-Bromothymol (39),
p-Bromothymol (41),
Dibromothymol (20)
Bromosalicylaldehyde (81),
Salicylaldehyde
85
75
98
24.8
22.7
28.5
Dibromosalicylaldehyde (19)
2-Bromo-4-hydroxybenzaldehyde (72),
2,6-Dibromo-4-hydroxybenzaldehyde (28)
2-Bromo-4-tert-butylphenol (65),
2,6-Dibromo-4-tert-butylphenol (35)
2-Bromo-4,6-dimethylphenol (100)
2,6-Dimethyl-4-bromophenol 100)
2,4-Di-tert-butyl-6-bromophenol (100)
2-Bromo-4-methyl-6-tert-butylphenol (100)
2-Hydroxy-3,5-di-tert-butyl-4,6-dibromo-benzaldehye (100)
2-Bromo-4,6-dichlorophenol (100)
4-Hydoxy-benzaldehyde
4-tert-Butylphenol
2,4-Dimethyl-phenol
2,6-Dimethyl-phenol
2,4-Di-tert-butylphenol
2-tert-Butyl-4-methyl-phenol
2-Hydroxy-3,5-Di-tert-butylbenzaldehye
2,4-Dichlorophenol
100
83
96
95
86
88
29.1
24.2
28.0
27.7
25.1
25.6
Table 5. Reaction time and TOF (sÀ 1) values for the oxidative bromination of phenol(s) with different catalysts.
S.
Substrate Reaction time [h] Maximum Ref.
no. Catalyst
TOF [sÀ 1
]
1
2
3
4
Enzymatic catalysis
Thymol
Thymol
Thymol
Different
Phenols
Thymol
Thymol
24
24
2
0.41
0.004
0.68
0.48
[34]
[34]
[35]
[36]
Chemical catalysis using NH4VO3
Cs(H2O)[VVO2{3,5-bis(2-hydroxyphenyl)-1-phenyl-1,2,4-triazole}]
[VVO2(Hdhap-inh)] and related[a]
2
5
6
7
(HNEt3)[VO2(L)][b]
2
2
1
1.8
1.0
24
[37]
[38]
[18]
[VVO(acac){6,6’-{2-(pyridin-2yl)ethylazanediyl}bis(methylene)-bis(2,4-di-tertbutylphenol)}]
Oxidovanadium(iv) β-octabromo-meso-tetrakis(2,6-dibromo-3,5-dimethoxyphenyl)porphyrin Different
phenols
Different
phenols
8
Oxidovanadium(iv) β-octabromo-meso-tetraphenyl porphyrin
0.5
29
This work
[a] Hdhap-inh=Schiff base derived from 2,4-dihydroxyacetophenone and isonicotinoylhydrazide. [b] H2L=Schiff base derived from 5-(arylazo)
salicylaldehyde and benzoylhydrazide.
match with their pure samples depicting their good chemical Conclusions
and thermal stabilities (Figures S13 and S15). We have done
three such recoveries of both catalysts and the efficiency of the
recovered catalysts (1 and 2) was examined for the oxidative
bromination of phenol under conditions similar to those used
for first cycle. The recycled [VIVO(TPP)] exhibited 94% con-
version of phenol with a TOF of 14.2 sÀ 1. Similarly, the recycled
[VIVO(TPPBr8)] exhibited a conversion of 95% and a TOF of
27.3 sÀ 1 with phenol. The minor loss of ca. 2–4% in catalytic
efficiency after each recovery of both catalysts can be explained
on the basis that the recovery processes, especially in
homogeneous catalysis, are not 100% efficient, which inevitably
leads to a small loss of catalyst.
In summary, the present work describes the first report on the
facile synthesis of [VIVO(TPPBr8)] (2) in nearly quantitative yield
(96%) by the selective self-oxidative bromination of [VIVO(TPP)]
(1) in the presence of HClO4/KBr/H2O2 as brominating agent in
waterÀ CH2Cl2 at room temperature under mild conditions. The
crystal structure of
2 exhibited nonplanar saddle shape
conformation with high ΔCβ (�1.1615 Å) and Δ24 (0.599 Å)
values supporting its robust structure. Complexes 1 and 2 were
utilized for the oxidative bromination of phenol and its
derivatives in water with extremely high TOF values (22.7–
29.1 sÀ 1 for 2) which are better than the TOF (0.4 sÀ 1) of
vanadium-dependent bromoperoxidase enzyme (VBPO) under
mild reaction conditions.[34] Additionally, 1 and 2 were success-
fully recovered at least three times after catalytic reactions and
reused for the same reactions. The current methodology helps
to overcome the multistep synthesis of brominated vanadyl
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porphyrins and their effective utilization in oxidative bromina-
tion catalysis. The electron withdrawing β-bromo substituents
as well as non-planar structure of 2 increases its stability during
catalytic processes and make it more suitable for the oxidative
bromination process even with its lesser amount compared to
1. It was found that electron donating group(s) enhance(s) the
reactivity of phenol which in turn influence on the relatively
higher yield of products and high turnover frequecy for electron
rich phenols.
Synthesis of
β-Octabromo-meso-tetraphenylporphyrinatooxidovanadium
(IV), [VIVO(TPPBr8)] (2)
To a solution of [VIVO(TPP)][18] (34.0 mg, 0.05 mmol) dissolved in
dichloromethane (2 mL), an aqueous solution (5 mL) of KBr
(297.5 mg, 2.5 mmol), 30% aqueous solution of H2O2 (283.3 mg,
0.253 mL, 2.5 mmol) and 70% aqueous solution of HClO4 (143.5 mg,
0.086 mL, 1.0 mmol) were added. The mixture was slowly stirred at
°
25 C for 36 h and the reaction was monitored through TLC and UV-
visible spectroscopy. The organic phase was diluted in dichloro-
methane (10 mL), washed with saturated NaHCO3 solution followed
by water. The organic layer was finally separated and dried over
Na2SO4. This process was repeated three times. After removing the
solvent under reduced pressure, the residue was purified by basic
alumina column using chloroform as eluent. Yield: 96% (63.0 mg,
0.048 mmol). Anal. Calcd for C44H20N4Br8VO (1310.4) C, 40.32; H,
1.54; N, 4.27. Found C, 40.45; H, 1.66; N, 4.34. UV-vis (CHCl3) λmax
Experimental Section
Materials and Methods
VOSO4 ·H2O (HiMedia, India), acetylacetone, salicylaldehyde, phenol,
thymol, p-cresol (Aldrich Chemical, USA), silica gel (100–200 mesh)
(Rankem, India) and bromine (SD fine chemicals, India) were of
analytical reagent grade and used as supplied. Other analytical
reagent grade chemicals and solvents used in this study were
procured from standard sources. Meso-tetraphenylporphyrin
(H2TPP) was prepared using Adler-Longo method[32] and the
insertion of VO2+ was carried out as explained in the reported
procedure.[16] The procedure for the direct oxidative bromination of
[VIVO(TPP)] (1) is described in SI.
(nm) (log ɛ): 362 (4.34), 464 (5.37), 590 (4.24), 628 (4.18). IR/cmÀ 1
:
1028 (V=O). 1H NMR (500 MHz, CDCl3): 8.30 (bs, 8H, meso-o-Ph), 7.93
(s, 8H, meso-m-Ph), 7.66 (s, 4H, meso-p-Ph). MALDI-TOF-MS (m/z):
found 1310.035 [M]+, calcd 1310.44. EPR spectral data of 2: gk =
1.959; g? =1.987; Ak =167.1×10À 4 cmÀ 1; A? =74.4×10À 4 cmÀ 1
.
X-ray Crystal Structure Determination.
Three-dimensional X-ray data for [VIVO(TPPBr8)] (2) were collected
on a Bruker APEX-II CCD diffractometer at 100(2) K, using a graphite
monochromator and Mo-Kα radiation (λ=0.71073 Å) by the ϕ-ω
scan method. Reflections were measured from a hemisphere of
data collected of frames each covering 0.3 degrees in ω. A total of
27980 reflections were measured, all of which were corrected for
Lorentz and polarization effects and for absorption by semi-
empirical methods based on symmetry-equivalent and repeated
reflections. Of the total, 1620 independent reflections exceeded the
significance level jFj/σ(jFj) >4.0. After data collection, in each case
a multi-scan absorption correction (SADABS)[39] was applied, and
the structure was solved by direct methods and refined by full
matrix least-squares on F2 data using olex2-1.3 suite of
programs.[40,41] The non-hydrogen atoms were refined with aniso-
tropic thermal parameters in all cases. Hydrogen atoms were
included in calculation positions and refined in the riding mode. A
final difference Fourier map showed a residual density outside next
to C(7), which could not be refined: 1.17/–1.62 e.ÅÀ 3. A weighting
Instrumentation and Characterization Techniques
Column Chromatography was performed on silica gel (Rankem
laboratory, 100–200 mesh). TLC was performed on aluminium-
baked silica plates (contain-13% CaSO4 ·1= H2O, Silica gel-G/UV-254),
2
which were visualized by naked-eye. UV-visible spectra were
recorded using Shimadzu UV-visible spectrometer (UV-3600). IR
spectra were recorded as KBr pellets in the mid IR range of 4000–
400 cmÀ 1 on a Bruker spectrometer. MALDI-TOF spectra were
recorded using a Bruker Ultrafle Xtreme-TN MALDI-TOF spectrom-
eter using HABA as a matrix. Elemental analysis was carried out
using an Elementar Vario EL II instrument. 1H NMR spectra were
recorded in CDCl3 at 298 K on Bruker Avance 500 MHz instrument.
Electron paramagnetic resonance spectra were recorded on a
Bruker EMX EPR spectrometer in toluene at 120 K. Cyclic voltam-
metric measurements were carried out using a CHI 620E instrument
in CH2Cl2 containing 0.1 M TBAPF6 as the supporting electrolyte
under an argon atmosphere. A three electrode assembly was used
consisting of a Pt disk working electrode, Ag/AgCl as a reference
electrode and a Pt-wire as a counter electrode. Gas-chromato-
graphic analysis was performed on a Shimadzu 2010 plus fitted
with a Rtx-1 capillary column (30 m×0.25 mm×0.25 μm) and an
FID detector. Relative peak areas of the substrate and respective
product(s) in the chromatogram were analysed and the percent
conversion of substrate and selectivity of products was calculated
from GC data using the formula:
scheme w=1/[σ2(Fo )+(0.06119 P)2 +64.588787 P] for, where P=(j
2
2
2
Fo j +2jFc j )/3, was used in the latter stages of refinement. The
[VO] framework is displaced and occupies two different positions
respect to the center and to the plane formed by the four nitrogen
atoms of the porphyrin, due to the presence of two rotoinversion
3
1
1
3
3
axis (4-fold, symmetry operations: = +y, = -x, = -z and = +y, = -x,
4
4
4
4
4
3= -z) characteristics of the tetragonal space group, I41/a. This can
4
be interpreted as a disorder caused by the non-planarity of
porphyrin, in which each part occupies 0.50 of the positions.
Crystal data of 2 at 100(2) K: C44H20N4OVBr8, Formula weight=
1310.85, black plates, crystal dimensions: 0.1×0.07×0.03 mm,
crystal system: tetragonal, Space group: I41/a, wavelength=
0.71073 Å, a=20.4850(9) and c=10.3361(8) Å, V=4337.4(4) Å3, Z=
4, F000 =2487.5, dcalc =2.007 gcmÀ 3, μ=7.644 mmÀ 1, θ=2.21 to
26.4 , Rint =0.1075, Goodness-of-fit on F2 =1.040, R1 (Σj jFo j–jFc j j
°
/ΣjFo j)=0.0535, wR2 (all data) ({Σ[w(j jFo j –jFc j j)2]j/Σ[w(Fo )2]}
2
2
2
1/2
The identity of the products was confirmed using a GC-MS model
Perkin-Elmer, Clarus 500 and comparing the fragments of each
product with the electronic library available.
)=0.1386, largest differences peak and hole=1.17/–1.62 eÅÀ 3
.
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Acknowledgements
MRM and MS thank the Science and Engineering Research Board
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Keywords: Oxidovanadium(IV) prophyrin · Crystal structure ·
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Manuscript received: February 9, 2021
Revised manuscript received: March 18, 2021
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