Synthesis, characterization, X-ray crystal structure, DFT calculations, and catalytic properties of a dioxidovanadium(V) complex derived from oxamohydrazide and pyridoxal: A model complex of vanadate-dependent bromoperoxidase

Article abstract of DOI:10.1021/ic501216d

A vanadium(V) complex with the formula [Et₃NH][V^VO₂(sox-pydx)] with a new tridentate ligand 2-[2-[[3-hydroxy-5-(hydroxymethyl)-2-methylpyridin-4-yl]methylene]hydrazinyl]-2-oxoacetamide (soxH-pydxH), obtained by condensation of oxamohydrazide and pyridoxal (one of the forms of vitamin B₆), has been synthesized. The compound was characterized by various analytical and spectroscopic methods, and its structure was determined by single-crystal X-ray diffraction technique. Density functional theory (DFT) and time-dependent DFT calculations were used to understand the electronic structure of the complex and nature of the electronic transitions observed in UV-vis spectra. In the complex, vanadium(V) is found to be pentacoordinated with two oxido ligands and a bianionic tridentate ONO-donor ligand. The vanadium center has square-pyramidal geometry with an axial oxido ligand, and the equatorial positions are occupied by another oxido ligand and a phenolato oxygen, an imine nitrogen, and a deprotonated amide oxygen of the hydrazone ligand. A DFT-optimized structure of the complex shows very similar metrical parameters as determined by X-ray crystallography. The O₄N coordination environment of vanadium and the hydrogen-bonding abilities of the pendant amide moiety have a strong resemblance with the vanadium center in bromoperoxidase enzyme. Bromination experiments using H₂O₂ as the oxidizing agent, with model substrate phenol red, and the vanadium complex as a catalyst show a remarkably high value of k_cat equal to 26340 h^-1. The vanadium compound also efficiently catalyzes bromination of phenol and salicylaldehyde as well as oxidation of benzene to phenol by H₂O₂.

Full text of DOI:10.1021/ic501216d

Article
pubs.acs.org/IC
Synthesis, Characterization, X‑ray Crystal Structure, DFT Calculations,
and Catalytic Properties of a Dioxidovanadium(V) Complex Derived
from Oxamohydrazide and Pyridoxal: A Model Complex of
Vanadate-Dependent Bromoperoxidase
Chandrima Das,† Piyali Adak,† Satyajit Mondal,† Ryo Sekiya,‡ Reiko Kuroda,‡ Serge I. Gorelsky,*,⊥
and Shyamal Kumar Chattopadhyay*,†
†Department of Chemistry, Indian Institute of Engineering Science and Technology, Shibpur, Howrah 711 103, India
‡Department of Life Sciences, Graduate School of Arts and Sciences, The University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902
Japan
⊥Centre for Catalysis Research and Innovation, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5,
S
* Supporting Information
ABSTRACT: A vanadium(V) complex with the formula
[Et3NH][VVO2(sox-pydx)] with a new tridentate ligand 2-[2-[[3-
hydroxy-5-(hydroxymethyl)-2-methylpyridin-4-yl]methylene]-
hydrazinyl]-2-oxoacetamide (soxH-pydxH), obtained by conden-
sation of oxamohydrazide and pyridoxal (one of the forms of
vitamin B6), has been synthesized. The compound was
characterized by various analytical and spectroscopic methods,
and its structure was determined by single-crystal X-ray diffraction
technique. Density functional theory (DFT) and time-dependent
DFT calculations were used to understand the electronic structure
of the complex and nature of the electronic transitions observed in
UV−vis spectra. In the complex, vanadium(V) is found to be
pentacoordinated with two oxido ligands and a bianionic tridentate
ONO-donor ligand. The vanadium center has square-pyramidal
geometry with an axial oxido ligand, and the equatorial positions are occupied by another oxido ligand and a phenolato oxygen,
an imine nitrogen, and a deprotonated amide oxygen of the hydrazone ligand. A DFT-optimized structure of the complex shows
very similar metrical parameters as determined by X-ray crystallography. The O4N coordination environment of vanadium and
the hydrogen-bonding abilities of the pendant amide moiety have a strong resemblance with the vanadium center in
bromoperoxidase enzyme. Bromination experiments using H2O2 as the oxidizing agent, with model substrate phenol red, and the
vanadium complex as a catalyst show a remarkably high value of kcat equal to 26340 h−1. The vanadium compound also efficiently
catalyzes bromination of phenol and salicylaldehyde as well as oxidation of benzene to phenol by H2O2.
monoperoxovanadium(V) complex is formed, where η2-
INTRODUCTION
■
peroxide, Nε (histidine), and oxido/hydroxo groups occupy
The discovery of vanadium-dependent bromoperoxidase in
marine algae,1−3 some lichens,4,5 and fungi,6−12 which is used
the square plane and an oxido ligand occupies the axial site
(Chart 1).24,27−29 There is extensive hydrogen bonding
for the catalytic synthesis of organobromo compounds, has
evoked interest in model studies of this enzyme.13−23 It is
between the coordinated oxido/hydroxo and peroxo ligands
and amino acid residues from protein side chains.23,24,30
A
believed that the organobromo natural products are used by the
marine organisms for self-defense against predators.3 Besides,
the organobromo compounds are also known to possess
antimicrobial and anticancer properties, which make them
potential candidates for medicinal applications.3,24,25 It is now
established that in the resting state the bromoperoxidase
enzyme contains vanadium(V) in a trigonal-bipyramidal
coordination geometry with three nonprotein oxido groups in
the trigonal plane and axial coordination by Nε of a histidine
imidazole and an OH group.6,7,24,26 In the active state, which
requires the presence of H2O2, a square-pyramidal
change of the oxidation state of the vanadium center has not
been observed during the catalytic turnover.31−33 The
vanadium(V) center acts as a strong Lewis acid to activate
the peroxo group, which then undergoes nucleophilic attack by
bromide to form the reactive “Br+” intermediate (most likely a
HOBr/Br2/Br3 species).31,34 The reactive “Br+” intermediate,
−
in turn, electrophilically attacks suitable organic substrates to
Received: May 24, 2014
Published: October 16, 2014
© 2014 American Chemical Society
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Chart 1. Vanadium Chloroperoxidase Enzyme in Its Native Form and in a Peroxo-Bound Form8,27,36
and the pH was set to 6.5−7.0 by the addition of concentrated KOH.
Oxamohydrazide (semioxamazide; 0.206 g, 2 mmol) dissolved in water
(5 mL) was added drop by drop to the pyridoxal suspension. The
solution was stirred for 3 h, and a yellow precipitate was obtained. The
residue was filtered and washed with ice-cold methanol and diethyl
ether. Yield: 472 mg (94%). Anal. Calcd for C10H12N4O4 (MW
252.23): C, 47.60; H, 4.76; N, 22.21. Found: C, 47.35; H, 4.83; N,
form brominated organic compounds. The peroxovanadium(V)
form of the related chloroperoxidase enzyme has been
structurally characterized for the fungi Curvularia inaequa-
lis.8,35−37 Single-crystal X-ray structures of quite a few model
peroxovanadium(V) complexes capable of oxidative halogen-
ation in the presence of H2O2 are also known.30−32,34,38 The
peroxo form of the enzyme is also capable of enantioselective
oxidation of organic sulfides to sulfoxides.39−43
Vanadium complexes are also capable of oxidizing a variety of
other organic substrates, e.g., epoxidation of alkenes and allyl
alcohols, hydroxylation of alkenes and arenes, oxidation of
primary and secondary alcohols to corresponding aldehydes
and ketones, etc.44,45 Although quite a few structural and/or
functional model complexes of the bromoperoxidase enzyme
are known in the literature,27,30−32,34,44−50 it still appears that
an understanding of the structural and electronic factors needed
for predicting the stability and activity of model vanadium
complexes during the catalytic cycle is lacking. Hence, a search
for new vanadium complexes that mimic the enzyme activity
and an understanding of their key structural and electronic
features are very much desired. In this paper, we report a new
vanadium(V) complex that exhibits bromoperoxidase activity
with phenol, phenol red, and salicylaldehyde as substrates. The
catalytic activity of the complex for oxidation of benzene to
phenol by H2O2 is also reported.
1
22.40. H NMR (DMSO-d6): δ 12.85 (1H, s), 12.04 (1H, s), 9.09
(1H, s), 8.39 (1H, s), 8.02 (2H, d), 5.37 (1H, s), 4.59 (2H, s), 2.42
(3H, s) (Figure S1 in the Supporting Information, SI). 13C NMR
(DMSO-d6): δ 161.45, 157.34, 151.33, 149.45, 147.43, 137.89, 133.59,
120.87, 58.90, 18.71 (Figure S2 in the SI). Electronic spectrum in a
N,N-dimethylformamide (DMF) solution with λmax/nm (εmax/M−1
cm−1): 340 (5110), 296 (10340). Selected IR bands (cm−1): 3412
(νasymNH ), 3258 (νsymNH ), 3165 (νO−H), 3041 (νN−H), 1681 (νCO),
2
2
1595 sh (νCN).
Synthesis of Complex [Et3NH][VVO2(sox-pydx)] (1). To a
vigorously stirred methanolic solution (15 mL) of the ligand soxH-
pydxH (251 mg, 1 mmol) was added Et3N (202 mg, 2 mmol). A
solution of VOSO4·H2O (180 mg, 1 mmol) in methanol (10 mL) was
added dropwise to the above solution. The solution turned chocolate
brown. The mixture was stirred at room temperature for 3 h and then
filtered. The yellowish-brown residue, containing the crude complex,
was washed with cold methanol and then dried over fused calcium
chloride. The complex was recrystallized from methanol. The filtrate
was allowed to evaporate slowly at room temperature. After 2 days,
yellow, square-shaped, shiny crystals of complex 1 suitable for X-ray
diffraction studies were obtained. Yield: 356 mg (82%). Anal. Calcd for
C16H26N5O6V (MW 435.36): C, 44.10; H, 5.97; N, 16.08. Found: C,
44.07; H, 5.95; N, 15.98. ESI-MS (positive-ion mode): m/z 378.78
[({VO2(sox-pydx)} + 2Na)+ (100%)] (Figure S3 in the SI). ESI-MS
(negative-ion mode): m/z 333.14 [{VO2(sox-pydx)}− (100%)]
EXPERIMENTAL SECTION
■
Physical Methods and Materials. Pyridoxal hydrochloride,
oxamohydrazide (semioxamazide), and VOSO4·H2O were obtained
from Aldrich, and triethylamine was purchased from E. Merck. All
other chemicals and solvents were of reagent grade and were used as
such, while solvents for spectroscopic and cyclic voltammetry (CV)
studies were of high-performance liquid chromatographic grade
obtained from Merck or Aldrich. Elemental analyses were performed
on a PerkinElmer 2400 carbon, hydrogen, and nitrogen analyzer. IR
spectra were recorded as KBr pellets on a Jasco FT-IR-460
spectrophotometer. UV−vis spectra were recorded using a Jasco V-
530 spectrophotometer. CV and differential pulse voltammetry (DPV)
experiments were carried out using a CH1106A potentiostat. A three-
electrode configuration, with a glassy carbon working electrode, a
platinum auxiliary electrode, a Ag/AgCl-saturated KCl reference
electrode, and TEAP as the supporting electrolyte, was used. Under
our experimental conditions, a ferrocene/ferrocenium couple was
1
(Figure S4 in the SI). H NMR (DMSO-d6): δ 9.28 (1H, s), 7.75
(2H, s), 7.55 (1H, s), 5.40 (1H, s), 4.66 (2H, s), 3.09 (7H, s), 2.34
(3H, s), 1.16 (9H, s) (Figure S4 in the SI). Electronic spectrum in a
DMF solution with λmax/nm (εmax/M−1 cm−1): 325 (9770), 411
(5800). Selected IR bands (cm−1): 3468 (νasymNH ), 3272 (br, νsym
NH2
2
+ νO−H), 3022 (νN−H),1690 (νCO), 1612 (νCN), 1446 (νC−O), 899
and 953 (νOVO) (Figure S6 in the SI).
X-ray Crystallography. Single-crystal X-ray data were collected
for 1 on a Bruker SMART APEXCCD diffractometer at 103(2) K,
using Mo Kα radiation (0.71073 Å). Data were corrected for
absorption effects using the numerical method SADABS,51 solved by
direct methods, and refined by full-matrix least squares on F2 using
SHELX-97.52 The non-hydrogen atoms were refined with anisotropic
displacement parameters. All hydrogen atoms were placed at
calculated positions and refined as riding atoms using isotropic
displacement parameters. A summary of the crystallographic data is
presented in Table 1. Important bond distances and bond angles are
collected in Table 2. Crystallographic data have been deposited at the
Cambridge Crystallographic Data Centre as deposition number
1004534.
1
observed at E0 (ΔEp) = 0.48 V (100 mV). H NMR spectra were
recorded on a Bruker AVANCE DPX 300 MHz spectrometer using
Si(CH3)4 as the internal standard. Electrospray ionization mass
spectrometry (ESI-MS) spectra of the samples were recorded on a
JEOL JMS 600 instrument. The products of the catalytic reactions
were identified and quantified by using a Varian CP-3800 gas
chromatograph equipped with a flame ionization detector.
Density Functional Theory (DFT) Calculations. All DFT
calculations were performed using the Gaussian 09 package53 using
the B3LYP hybrid functional54 and the TZVP basis set55 for all atoms.
The structures of all species were optimized without any constraints in
Synthesis of the Ligand 2-[2-[[3-Hydroxy-5-(hydroxymeth-
yl)-2-methylpyridin-4-yl]methylene]hydrazinyl]-2-oxoaceta-
mide (soxH-pydxH). To prepare the tridentate ligand, pyridoxal
hydrochloride (0.406 g, 2 mmol) was dissolved in 40 mL of methanol
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calculations were carried out using the AOMix program.58d,e Time-
dependent DFT (TD-DFT) calculations were performed to calculate
the absorption spectra. These spectra were simulated with the SWizard
program,59 revision 4.6, using pseudo-Voigt functions with a half-
bandwidth, Δ1/2, of 3000 cm−1.
Table 1. Crystal Data and Refinement Details of 1
formula
fw
C16 H26 N5 O6V
435.36
cryst size (mm3)
temperature (K)
cryst syst
space group
a (Å)
0.37 × 0.24 × 0.07
103(2)
Catalytic Study. The bromination of phenol red to bromophenol
blue using complex 1 as a catalyst was studied by absorption
spectroscopy. An aliquot of 30% H2O2 was added to a DMF solution
of 0.1 mmol of catalyst, so that the final concentration of H2O2 in the
reaction mixture was 1 mM. This was followed by the addition of
HClO4 (final concentration 2.0 mM) and 4.0 mmol of tetra-N-
butylammonium bromide. Finally, 0.25 mmol of phenol red was
added, and the solution was stirred at a constant temperature of 25 °C.
Spectral measurements of aliquots withdrawn, and buffered at pH =
7.1 with an aqueous phosphate buffer, were taken at regular time
intervals. The spectral data show the gradual disappearance of the 434
nm band and growth of the band of bromophenol blue at 590 nm, and
the reaction was complete under the ambient conditions after ∼4 h.
Kinetic studies of the bromination of phenol red to bromophenol
blue were carried out using complex 1 as a catalyst, by monitoring the
increase in absorbance at 590 nm by the initial rate method. To a
DMF solution of 0.01 mmol of catalyst were added under stirring 0.1
mM 30% H2O2, 0.2 mM HClO4, and 0.4 mmol of tetra-N-
butylammonium bromide, and the reaction mixture was thermostated
at 25 °C. With an increase of the substrate (phenol red) concentration
from 20 to 60 equiv with respect to the catalyst (0.01 mmol), the
dependence of the rates on the substrate concentration was
determined.
triclinic
P1
̅
8.2552(7)
10.1594(9)
12.645(1)
68.371(2)
83.963(2)
81.757(2)
1.484
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
dcal
Z
2
μ (mm−1
)
0.553
F(000)
456.0
total reflns
6018
unique reflns
4238
obsd data [I > 2σ(I)]
R(int)
3719
0.0286
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)
0.0509, 0.1295
0.0579, 0.1351
Table 2. Bond Distances (Å) and Angles (deg) for 1
bond lengths (Å)
V1−O1
V1−O3
V1−O4
1.653(2)
1.900(2)
1.989(2)
V1−O6
V1−N1
1.617(2)
2.156(2)
The dioxidovanadium complex 1 can also catalyze the oxidative
bromination of salicylaldehyde as well as phenol using H2O2, KBr, and
HClO4 in an aqueous solution. To achieve an optimum reaction
condition for the bromination reaction, the volumes of H2O2 and acid
were varied. With salicylaldehyde as the substrate, for a fixed amount
of substrate (5 mmol), 0.01 mmol of catalyst, 10 mmol of KBr, 30
mmol of H2O2, and 5 mmol of HClO4 in an aqueous medium were
found to be the optimum conditions, where the maximum amount of
the substrate is converted to the desired product 5-bromosalicylalde-
hyde. To stop decomposition of the catalyst, 1 mmol portions of
HClO4 were added successively, during the course of the reaction
every 1 h for 4 h.
For the catalytic study of oxidation of benzene by H2O2, aqueous
H2O2 (25 mmol) and benzene (50 mmol) were taken in 5 mL of
acetonitrile. Complex 1 (0.1 mmol) dissolved in a minimum volume of
water was added to it. The reaction mixture was thermostated at 80 °C
in an oil bath with stirring. After specific time intervals, small amounts
of aliquots were withdrawn from the reaction mixture and analyzed by
gas chromatography (GC).
bond angles (deg)
O1−V1−O3
O1−V1−O4
O3−V1−O4
O6−V1−O1
O6−V1−O3
96.13(8)
91.31(8)
O6−V1−O4
O1−V1−N1
O3−V1−N1
O4−V1−N1
O6−V1−N1
101.57(8)
143.57(8)
81.31(7)
73.59(7)
106.75(8)
146.48(8)
108.72(9)
106.77(9)
solution using water as a solvent and the SMD implicit solvation
model.56 The same level of theory with tight self-consistent-field
convergence criteria was used for all other calculations. The converged
wave functions were tested to confirm that they correspond to the
ground-state potential energy surface (PES). Evaluation of the atomic
charges and spin densities was performed using natural population
analysis.57 Analyses of the molecular orbitals in terms of fragment
orbital contributions (based on the Mulliken population analysis,
MPA)58a and Mayer58b and orbital occupancy perturbed bond order58c
Scheme 1. Synthesis of Complex 1
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ligand (see the TD-DFT section of the Results and Discussion
section). The zinc(II) complex of the same ligand shows a band
of comparable intensity at 427 nm (Figure S7 in the SI),
supporting the above assignment of the band as a ligand-
centered rather than a ligand-to-metal charge-transfer (LMCT)
transition. Another strong band at 325 nm corresponds to a
π(L) → dπ(V) LMCT transition (Figure 1).
RESULTS AND DISCUSSION
■
Synthesis and Spectroscopic Characterization. It has
been reported in the literature that the dioxidovanadium(V)
complex of pyridoxal isonicotinoyl hydrazone shows bromo-
peroxidase mimicking activity.46 Although no explanation was
offered in the paper as to why the complex showed such
activity, our own experience with pyridoxal Schiff base ligands60
prompted us to believe that the hydrogen-bonding ability of the
pyridoxal residue may play an important role in determining
the bromoperoxidase mimicking ability, by stabilizing the
catalyst−substrate complex. With this assumption, we thought
that a vanadium(V) complex with a pyridoxal Schiff base of
oxamohydrazide, with a pendant amide group, providing added
hydrogen-bonding sites but less steric bulk compared to the
pyridine ring of isonicotinoyl hydrazone, may be a better
candidate for the exploration of model bromoperoxidase
+
acitivity by complexation with VO2 . For this purpose, the
ligand soxH-pydxH was synthesized by reacting methanolic
solutions of neutral pyridoxal with oxamohydrazide at room
temperature. Upon reaction of the ligand with vanadium sulfate
in the presence of triethylamine in a methanol medium, the
monoanionic vanadium(V) complex 1 was obtained with
triethylammonium as the countercation (Scheme 1). In the
free ligand, a sharp, relatively broad band at 1681 cm−1 is
assigned to overlapping νCO vibrations of the two carbonyl
groups, whereas a shoulder at 1595 cm−1 is assigned to the
Figure 1. UV−vis spectra for the ligand soxH-pydxH and complex 1 in
DMF at 298 K.
νCN vibration. The asymmetric stretching vibration (νasym
)
NH2
of the NH2 moiety is located at 3412 cm−1. Between 3260 and
3000 cm−1, the free ligand shows a strong absorption envelope
with broad peaks at 3258, 3165, and 3041 cm−1, which are
Description of the Structure. The molecular structure of
the vanadium(V) complex is shown in Figure 2, and relevant
assigned to νsymNH , νO−H, and νN−H, respectively. The broad
2
envelope indicates extensive hydrogen bonding involving
multiple OH and NH functionalities. In the vanadium complex
1, a νasym
vibration is observed at 3468 cm−1, whereas the
NH2
relatively broad peak at 3272 cm−1 is assigned to overlapping
νsym
and νO−H (hydroxymethyl) vibrations. The bands at
NH2
3165 and 3041 cm−1 present in the free ligand are absent in the
complex, whereas a new band observed at 3022 cm−1 is
probably due to the νN−H vibration of the hydrogen-bonded
triethylammonium group. The νCO vibration of the pendant
amide is observed in the complex at 1690 cm−1, while the νCN
vibration is now located at 1612 cm−1. A new band at 1446
cm−1 in the complex is assigned to the νC−O vibration,31,32
while two strong vibrations at 899 and 953 cm−1 are assigned to
the antisymmetric and symmetric stretching vibrations of the
dioxovanadium(V) moiety (Figure S6 in the SI).
1
In the H NMR spectra, the phenolic OH proton at 12.85
ppm, present in the ligand, is absent in the complex, whereas
the aldimine proton undergoes a downfield shift from 9.09 ppm
in the ligand to 9.29 ppm in the complex. The aromatic proton
shifts upfield from 8.39 ppm in the ligand to 7.55 ppm in the
complex. The methyl and methylene protons attached to the
heterocyclic ring are observed at 2.42 and 4.59 ppm,
respectively, in the ligand and at 2.34 and 4.66 ppm,
respectively, in the complex.
The electronic spectrum of the ligand, recorded in a DMF
solution, consists of an intense band at 340 nm, assigned to the
π → π* transition of the imine moiety, and another more
intense transition at 296 nm, due to the π → π* transition of
the heterocyclic ring.60 In the vanadium(V) complex, a strong
band at 411 nm corresponds to the π → π* transition of the
Figure 2. X-ray crystal structure of [Et3NH][VO2(sox-pydx)]. For
clarity, only the anionic complex part is shown.
metal−ligand bond distances and angles are reported in Table
2. In the structure, the vanadium ion is in an O4N-donor
environment, arranged in a square-pyramidal geometry
(Addison parameter61 τ = 0.05; τ = 0.00 for a square pyramid
and 1.00 for a trigonal bipyramid). The equatorial plane
consists of O1, O3, O4, and N1 atoms, and the vanadium atom
is displaced by 0.5212(4) Å from the plane toward the axial
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oxygen O6. The metrical parameters are similar to those
reported in the literature for monoanioinic vanadium(V)
complexes in an O4N-donor environment, which includes
two oxido ligands.46,62 The metal−ligand distances follow the
order VO (O1 and O6) < V−O− (O3 and O4) < V−N. The
VO and V−N distances in the present complex compare well
with those of the choloroperoxidase enzyme from C. inaequalis,
where the corresponding bond distances are 1.65 and 2.25 Å,
respectively.8 The pyridoxal ring nitrogen and the pendant
hydroxymethyl group attached to it, as well as the
uncoordinated amide moiety, participate in the formation of
an extensive hydrogen-bonding network. The hydroxymethyl
O2−H1 forms a complementary hydrogen bond with the oxido
oxygen O1 of a neighboring unit (O2−H1···O1i; symmetry
code i = −x, −y, 1 − z; Table S1 in the SI) to form a hydrogen-
bonded dimer. Such dimers are then interconnected with each
other by N4−H4···N3ii (symmetry code ii = −1 + x, 1 + y, z)
by hydrogen bonds to form a one-dimensional chain along the
[1, −1, 0] base vector. The triethylammonium cations in the
crystal are hydrogen-bonded to the vanadium complex anions,
where the coordinated amide oxygen O4 and the pendant
amide oxygen O5 act as hydrogen-bond acceptors and the N5
of the triethylammonium ion acts as a hydrogen-bond donor
(Figure S8 in the SI).
Table 3. Energies and MPA-Derived Compositions of FMOs
of Complex 1 from DFT Calculations
% contribution
molecular
orbital
description
ε (eV)
−1.40
−1.57
V
Oax
9.6
Oeq
4.7
4.7
L
2
LUMO+3
LUMO+2
d (V)
69.2
63.1
16.5
16.7
z
dxz(V)−
15.5
px(Oax)
LUMO+1
dxy(V)−
−1.96
72.9
1.7
15.1
10.3
px(Oeq)
LUMO
π*(L) + dxz(V) −2.53
8.8
1.6
0.0
1.7
3.4
2.3
2.8
1.6
2.1
1.4
1.4
3.6
85.7
94.7
95.8
93.1
HOMO
π(L)
π(L)
σ(L)
−6.18
−6.77
−7.03
HOMO−1
HOMO−2
oxido and sox-pydx ligand contributions of these molecular
orbitals reflect on the covalent bonding between the metal and
ligands in this complex. As can be expected for {VO2}+
complexes, the V−Oax and V−Oeq interactions have the V
O description, and the calculated Mayer bond orders (1.87 for
V−Oax and 1.82 for V−Oeq) confirm this. The vanadium−sox-
pydx ligand covalent interaction is reflected by the V−O3, V−
O4, and V−N1 bond orders (0.60, 0.51, and 0.37, respectively),
with the V−N1 bond being the weakest metal−ligand
interaction in the complex.
Electronic Structure. The DFT-optimized structure of the
anionic [VVO2(sox-pydx)]− complex reproduces the coordina-
tion environment of the central atom in the X-ray structure of
the complex (Figure 2 and Table 2). The calculated VOax
and VOeq bond lengths are 1.631 and 1.639 Å, respectively,
while the calculated V−O3, V−O4, and V−N1 bond lengths
are 1.930, 2.015, and 2.209 Å, respectively. The Oeq−V−Oax
angle in the calculated structure (109.7°) also agrees well with
the experimental value (108.7°). The lowest-energy structure
from the DFT optimization contains the uncoordinated amide
moiety (atoms C10, N4, and O5), which is rotated 180° around
the C9−C10 bond relative to the X-ray structure. The energy
minimum that contains the amide moiety with the same
orientation as that in the X-ray structure has a slightly higher
energy in solution (ΔE of 0.29 kcal mol−1 and ΔG298 K of 0.28
kcal mol−1). However, this geometry can be stabilized by
hydrogen bonding in the solid state, as observed in the X-ray
structure. The rotation of the amide moiety around the C9−
C10 bond has a low-energy barrier in solution (ΔE⧧ of 3.4 kcal
The calculated absorption spectrum of complex 1 in solution
(Table 4) contains two strong bands at 415 and 329 nm (411
Table 4. Calculated Lowest-Energy Absorption Bands for
Complex 1 in Solution from TD-DFT Calculations
λ
(nm)
f
assignment
415 0.102 HOMO → LUMO (92%)
385 0.002 HOMO → LUMO+1 (94%)
346 0.015 HOMO−2 → LUMO (78%)
344 0.064 HOMO−1 → LUMO (52%)
HOMO → LUMO+2 (21%)
π(L) → π*(L)
π(L) → dxy(V)−px(Oeq)
σ(L) → π*(L)
π(L) → π*(L)
π(L) → dxz(V)−px(Oax)
π(L) → dxz(V)−px(Oax)
π(L) → dxy(V)−px(Oeq)
329 0.154 HOMO → LUMO+2 (50%)
321 0.017 HOMO−1 → LUMO+1
(41%)
2
HOMO → LUMO+3 (30%)
π(L) → dz (V)
2
319 0.056 HOMO → LUMO+3 (41%)
π(L) → dz (V)
mol−1 and ΔG⧧
of 3.8 kcal mol−1). Thus, the optimized
structure in solution is consistent with the experimental X-ray
structure of the complex in the solid state.
298 K
and 325 nm in the experimental spectrum in a DMF solution).
The lowest-energy absorption is due to the HOMO → LUMO,
π(L)→ π*(L) intraligand electronic excitation. The second
strong absorption band near 330 nm is due to the HOMO →
LUMO+2, π(L)→ dxz(V)−px(Oax) LMCT transition. There
are weaker transitions between the first and second strong
transitions and near the second strong transition at 329 nm.
These bands are expected to contribute to the absorbance in
the near-UV region, but they are not expected to show up as
distinct bands, being obscured by the stronger bands at 415 and
329 nm (Figure 3).
Electrochemical Studies. In the CV experiments, the
vanadium complex 1 shows two ligand-based oxidations at 1.2
and 1.6 V (Figure S10 in the SI): the oxidation at lower
potential probably corresponds to oxidation of the phenolate
oxygen, whereas at more positive potential, hydrazone-based
oxidation occurs. In the zinc(II) complex of the same ligand,
irreversible oxidative waves are observed at 0.95 and 1.5 V,
supporting the above assignments of the oxidative response of
The frontier molecular orbitals (FMOs) of complex 1 are
shown in Table 3 and Figure S9 in the SI. The three highest
occupied molecular orbitals of the complex are the π (the
HOMO and HOMO−1) and σ (HOMO−2) orbitals of the
sox-pydx ligand. The lowest unoccupied molecular orbital
(LUMO) of the complex is mostly π* of the sox-pydx ligand
(86%) with a small contribution (9%) from the dxz(V) orbital
(the Cartesian coordinate system is set up in such a way that
the Z axis points along the V−Oax vector and the X axis goes
along the O3−O4 direction). The orbital occupancy perturbed
Mayer bond order calculations58c indicate that the LUMO is a
weak antibonding orbital (Table 3). Above the LUMO lie three
metal-based orbitals: dxy(V)−px(Oeq) as LUMO+1, dxz(V)−
2
px(Oax) as LUMO+2, and a dz -like orbital as LUMO+3. The
metal contribution to these orbitals ranges from 63% in LUMO
+2 to 73% in LUMO+1, and all three orbitals have antibonding
character for the V−Oax and V−Oeq interactions (Table 3). The
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Table 5. Electrochemical Data for Complex 1
scan
rate
E0/V (ΔEp/mV) values from peak potentials (V) in
complex
CV
DPV
1
100
1.64, 1.21, −0.90, −1.45
1.47, 1.14, −0.81, −1.42
(100)
Figure 3. Simulated absorption spectra of complex 1, [VO(OH)(sox-
pydx)], and [VO(O2)(sox-pydx)]− in solution.
complex 1 to ligand-based redox processes. On the negative
side of the Ag/AgCl electrode, an irreversible reduction wave
was observed at −0.91 V followed by a quasi-reversible
reduction wave at −1.45 V, with a peak-to-peak separation of
100 mV. On the basis of the fact that DFT calculations have
shown (vide supra) that LUMO is predominantly ligand-
centered while LUMO+1 has major contribution from the
metal dxy orbital, the first irreversible reduction is assigned to a
ligand-centered process, while at more negative potential, the
quasi-reversible redox couple is assigned to a vanadium(V)/
vanadium(IV) redox pair (Figure 4 and Table 5). Similar results
were obtained for a monoanionic dioxidovanadium(V) complex
of 5-bromosalicylaldehyde semicarbazone containing vanadium-
(V) in an O4N-donor environment.62a
Reaction with HCl. The cis-dioxidovanadium(V) complex
gives an oxidohydroxo complex upon reaction with acids. The
spectral changes observed when 10 μL aliquots of a saturated
methanolic solution of HCl were added successively to 2 mL of
a 10−4 M methanolic solution of [Et3NH] [VO2(sox-pydx)] is
depicted in Figure 5. The addition of a HCl solution results in a
Figure 5. Titration of [Et3NH][VO2(sox-pydx)] with a saturated
solution of HCl in methanol. The spectra were recorded after the
gradual addition of 10 μL of methanolic HCl to 2 mL of a 1 × 10−4 M
methanol solution of [Et3NH][VO2(sox-pydx)].
color change of the complex from yellow to yellowish orange
with a slight broadening and decrease in the intensity of the
absorption band at 409 nm. Further addition of HCl results in
the disappearance of the 294 nm band and the appearance of a
new band at 272 nm. An increase in the intensity of the 227 nm
band is also observed (Figure 5). It has been established that,
upon acidification of the pyridoxal hydrazone complexes of
dioxidovanadium(V), an initial protonation takes place at the
Figure 4. CV and DPV of complex 1 in DMF. For DPV, the current axis is on the right-hand side of the graph.
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Scheme 2. Protonation of Complex 1
pyridine nitrogen,45,46,63−65 which is followed by protonation of
the hydrazone nitrogen (N2), and finally one of the oxido
groups coordinated to vanadium is protonated (Scheme 2).46
TD-DFT calculation on the hydroxo complex [VO(OH)(sox-
pydx)] also points to the effect of protonation on the first
strong absorption band at 409 nm (Figure 3). The hydroxo
complex has significantly lower absorption intensity in this
region relative to the dioxidovanadium(V) complex. Consider-
ing that the color change and spectral shifts observed by us are
similar to those of previous reports,45−48 we also assume that
the final species formed upon the addition of excess acid to the
vanadium complex [VO2(sox-pydx)]− is [VO(OH)(soxH-
pydxH)]2+. This transformation is reversible, and the spectra
of the parent complex can be regenerated upon the addition of
methanolic NaOH (Figure 6). For the catalytic activity of
vanadate-dependent haloperoxidase, the presence of a coordi-
nated hydroxo group as well as the reversibility of the dioxido
to oxidohydroxo complex conversion is an important
observation.46,65,66 In the protonated complex, the vanadium-
(V)/vanadium(IV) couple is observed at 30 mV more positive
potential than the parent complex (Figure S11 in the SI).
Reaction with H2O2. The addition of H2O2 to a methanolic
solution of the dioxidovanadium complex leads to formation of
the oxidoperoxo complex, which we tentatively assume is
[VO(O2)(sox-pydx)]−. Although we are unable to isolate the
oxidoperoxo complex in the solid state, its formation can be
established by absorption spectroscopy (Figure 7) and ESI-MS
Figure 7. Titration of [Et3NH][VO2(sox-pydx)] with 30% H2O2. The
spectra were recorded after successive additions of 10 μL of H2O2 to 2
mL of a 0.5 × 10−4 M solution of [Et3NH][VO2(sox-pydx)] in
methanol.
(Figures S3 and S4 in the SI). The changes recorded in the
absorption spectrum after successive additions of 10 μL of 30%
H2O2 to 2 mL of 0.5 × 10−4 M methanolic [Et3NH][VO2(sox-
pydx)] are shown in Figure 7. A slight broadening and decrease
in the intensity of the absorption maximum at 409 nm are
observed after the addition of H2O2. Further addition of H2O2
leads to an increase in the intensity of the band at 336 nm along
with a blue shift to 324 nm (Figure 7). The ESI-MS spectra
(both positive- and negative-ion mode) also show a new peak
Figure 6. Titration of [VO(OH)(sox-pydx)] (0.5 × 10−4 M) with a
saturated solution of NaOH in methanol. The spectra were recorded
after the gradual addition of 20 μL of methanolic NaOH.
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Chart 2. Oxidative Bromination of Phenol Red to Bromophenol Blue Catalyzed by 1
Figure 8. [Et3NH][VO2(sox-pydx)] (0.1 mmol) catalyzed bromination of phenol red (0.25 mmol) followed by absorption spectroscopy. Spectral
data taken of aliquots in pH = 7.1 aqueous phosphate buffer.
assignable to [VO(O2)(sox-pydx)] (Figures S3 and S4 in the
SI). These results can be interpreted in terms of formation of
the oxidoperoxo complex of composition [VO(O2)(sox-
pydx)]−.45−48 We may note here that poor stability in the
solid state of peroxo complexes derived from oxido- and
dioxidovanadium(V) complexes with pyridoxal hydrazone and
thiohydrazone ligands has been reported in the literature.46,67
We have optimized the structure of the oxidoperoxovanadium-
(V) complex using DFT, and the optimized structure is given in
Figure S12 in the SI. The η2-peroxo complex was found to be
∼10 kcal mol−1 lower in energy (also a minimum in the PES)
than the corresponding η1-peroxo species.
Catalytic Activity Study. Oxidative Bromination. a. Con-
version of Phenol Red to Bromophenol Blue. The
bromoperoxidase activity of the enzymes as well as their
model complexes is often measured by the kinetic study of the
catalytic bromination of phenol red to bromophenol
blue.25,31,32
points at 482 and 389 nm, show clean conversion of phenol red
to bromophenol blue, and the reaction was complete after ∼4 h
under ambient conditions.
The plots for the kinetic studies of the above conversion are
given in Figures 9 and S13 and S14 in the SI. The reaction
showed saturation kinetics, and a treatment based on the
Michaelis−Menten model originally developed for enzyme
kinetics was carried out. The values of the Michaelis binding
constant (Km), maximum velocity (Vmax), and rate constant for
dissociation of substrates (i.e., turnover frequency, kcat) were
calculated from the Lineweaver−Burk graph (double reciprocal
plot) of 1/rate versus 1/[S] (Figure 9), using the equation 1/V
= (KM/Vmax)(1/[S]) + 1/Vmax, and the kinetic parameters are
presented in Table 6. The catalytic parameters (Km,Vmax, and
kcat) were also evaluated by putting the data into the
Michaelis−Menten kinetic model using a nonlinear least-
squares fitting routine, which gave almost identical values
(Table S2 in the SI). A control reaction carried out without the
vanadium complex shows no change in the absorbance at 590
nm (Figure S15 in the SI).
The catalytic ability of complex 1 for the oxidative
bromination of phenol red (Chart 2) was monitored by UV−
vis spectrophotometry (Figure 8). The disappearance of the
peak due to phenol red at 424 nm and the appearance of a new
peak at 590 nm due to bromophenol blue, with isosbestic
b. Bromination of Other Organic Substrates. Oxidative
brominations of salicylaldehyde and phenol, catalyzed by the
present dioxidovanadium complex (Chart 3), were also studied.
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Figure 9. Initial rates versus substrate concentrations for the bromination reaction (Chart 2) catalyzed by complex 1. The inset shows the
Lineweaver−Burk plot.
Table 6. Kinetic Parameters for the Bromination of Phenol
Red by Complex 1
Table 7. Bromination of Hydroxyaromatic Compounds
Using Complex 1 as a catalyst
Vmax (M min−1
4.39 × 10−3
)
Km (M)
4.4 × 10−4
kcat (h−1
)
yield
(%)
TOF
(h−1
a
substrate
major product
TON
)
26340
salicylaldehyde
phenol
5-bromosalicylaldehyde
71
29
355
145
88.75
36.25
p-bromophenol
In the case of salicylaldehyde, GC analysis revealed the
presence of three products in the ratio 71:4:25, with 5-
bromosalicylaldehyde as the major product and 3,5-dibromo-
salicylaldehyde as the other relevant product (4%) along with
an unidentified product (25%). For phenol, the selectivity of
the formation of p-bromophenol was 29% (Table 7). In the
absence of the catalyst, no brominated product was obtained
from the reaction mixture. Considering the high percentage of
substrate conversion, the relatively high selectivity of the
formation of 5-bromosalicylaldehyde, the substantially high
value of the turnover frequency (TOF), and also the fact that
our vanadium complex works in an aqueous medium, it is
probably one of the most efficient homogeneous catalysts
containing vanadium for oxidative bromination so far reported
in the literature.32,44−46,50,51 A plausible mechanism of the
reaction is shown in Scheme 3.
a
Time of reaction: 4 h.
several pendant groups, like hydoxymethyl and amide, that, like
the amino acid residues in the enzyme, are capable of stabilizing
the catalyst−substrate complex by hydrogen-bonding inter-
actions with the hyrdoxy aromatic substrates (similar to the
hydrogen-bonding interactions with Et3NH+ in the solid state
observed in the X-ray structure). Moreover, from the DFT-
optimized structure of the oxidoperoxo complex (Figure S12 in
the SI), it is clear that the hydroperoxo intermediate generated
during the catalytic cycle (Scheme 3), which is important for
increasing the electrophilicity of the peroxido complex,
activating the attack by Br−,31 may also be stabilized by
hydrogen-bonding interactions with either the phenolato or
amido oxygen.
One possible reason for the high catalytic efficiency of
complex 1 for the oxidative bromination reaction is probably
due to the fact that complex 1 not only has a donor
environment around vanadium(V) similar to that of the
vanadium-dependent bromoperoxidase enzyme but also has
Oxidation of Benzene to Phenol. Model complexes
showing haloperoxidase activity are also known to catalyze in
vitro oxidations of several other organic substrates.68 The
dioxido complex has also been employed to study the oxidation
of benzene to phenol (Chart 4). GC analysis revealed that in 18
Chart 3. Oxidative Bromination of Phenol and Salicylaldehyde Catalyzed by Complex 1
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Scheme 3. Plausible Mechanism of Oxidative Bromination Catalyzed by Complex 1
salicylaldehyde and phenol. It is also able to oxidize benzene
to phenol.
Chart 4. Oxidation of Benzene to Phenol with H2O2
Catalyzed by Complex 1
ASSOCIATED CONTENT
* Supporting Information
X-ray crystallographic data in CIF format, mass, H NMR, and
■
S
1
IR spectra, FMOs of complex 1. and other characterization
data. This material is available free of charge via the Internet at
http://pubs.acs.org.
h 78% benzene is converted to phenol. No further oxidation
was observed beyond 18 h (Table 8). Again, the high
AUTHOR INFORMATION
Corresponding Authors
*E-mail: sgorelsk@uottawa.ca.
■
Table 8. Oxidation of Benzene to Phenol Using Complex 1
as a Catalyst
*E-mail: shch20@hotmail.com. Fax: +91 033 2668 2916.
Notes
The authors declare no competing financial interest.
time (h)
conversion (wt %)
TON
TOF (h−1
)
3
6
29
47
72
78
145
235
360
390
48
39
40
22
9
ACKNOWLEDGMENTS
■
18
C.D. and P.A. thank the UGC for fellowships, while S.M.
thanks the CSIR. S.K.C. acknowledges AICTE for funding the
purchase of a CH1106A potentiostat. DST-FIST, UGC-SAP,
and MHRD special grants that funded the infrastructural facility
created in our department are also thankfully acknowledged.
We thank Prof. M. Ali of Jadavpur University for kindly
recording the ESI-MS spectra.
percentage of conversion of the substrate, as well as the high
TON and TOF values, indicates that complex 1 is also a very
efficient oxidation catalyst for the hydroxylation of aromatic
hydrocarbons by H2O2.
CONCLUSIONS
■
REFERENCES
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with the presence of pendant amide and hydroxymethyl groups,
which are capable of participating in hydrogen bonding with the
peroxo group or substrates, makes the compound a suitable
functional model of the enzyme. The compound, in fact, is
found to show exceptionally high bromoperoxidase activity,
with phenol red as a model substrate, as well as with
■
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